Prop Tunnels/Pockets on a 31

[andresvarro]

Anybody have any experience with prop tunnels on a Bertram 31?
Will be installing a pair from Cabrera. He has specified a distance between the centre of the strut supports on each tunnel to be 58-3/4". With this measurement, the shaft centreline is now extending closer to the gunnels than originally. The problem is that I require space to the side of the port engine to house the generator and with this measurement it may not fit anymore. Is there any guidelines as to how close tunnels can get to each other? Also I may have to modify the fuel tank if i do bring them closer together as they will now be fractionally inside the innermost stringers where the fuel tank rests.
I have Cummins 6BT 210s with 1-3/8" shafts, what props should I spin and any idea on blade tip clearance to tunnel??

[Tony Meola]

http://bertram31.com/newbb/viewtopic.ph ... ts#p112312

http://bertram31.com/newbb/viewtopic.ph ... ets#p70453
http://bertram31.com/newbb/viewtopic.ph ... ets#p76756
http://bertram31.com/newbb/viewtopic.ph ... ets#p68834
http://bertram31.com/newbb/viewtopic.ph ... ets#p16898

Andresvarro

Before you jump into the pockets, read carefully through the above threads. Not sure the work and cost is worth the gain. Running the 210's not sure what you would gain with pockets to justify all the work.

[andresvarro]

http://bertram31.com/newbb/viewtopic.ph ... ts#p112312

http://bertram31.com/newbb/viewtopic.ph ... ets#p70453
http://bertram31.com/newbb/viewtopic.ph ... ets#p76756
http://bertram31.com/newbb/viewtopic.ph ... ets#p68834
http://bertram31.com/newbb/viewtopic.ph ... ets#p16898

Andresvarro

Before you jump into the pockets, read carefully through the above threads. Not sure the work and cost is worth the gain. Running the 210's not sure what you would gain with pockets to justify all the work.

Yes, I've read some of the possible negative outcomes from installing pockets but out of my research so many people have talked about the advantages it could offer. My 31 had rotten stringers so I've gutted the whole boat. I'm basically starting from scratch and as a result the cost of installing the tunnels is negligible in the scheme of things. I've kept in mind that I may repower to the Yanmar 315s in the future so if it doesn't work out as fruitful as hoped, possibly later on, I could reap the benefits. Also installing fibreglass shaft tubes that will be able to accept a larger shaft using Capt Pats guidelines. Making sure everything is catered for if a bigger engine package gets installed.

[Carl]

Yes, I've read some of the possible negative outcomes from installing pockets but out of my research so many people have talked about the advantages it could offer.

You make a pretty strong argument for rethinking pockets.

I've heard about the advantages of prop pockets as have most here on the board. Problem is when all is said and done the boats just do not seem perform all that much better. Side note is when you take into account the time, effort, cost, lost resale value...

If I was to start from a blank canvas I would be looking very closely at Bob Lico's setup. Compare his real life numbers to those from 31's with pockets.
Seems dropping that motor angle is key...kinda what the pockets do in a backwards way. Other ways to achieve that...Look at Pete Fallons numbers...
Obviously its your boat, your time and wallet so you get to decide your direction...just feel its worthy of taking another closer look and try to compare real life outcomes...not base on peoples talk about possible advantages. It's a pair of big chunks being removed from the best hulls ever built...

Whatever you decide, hope you hang around here, love to see the progress pictures and outcome.
If you proceed with pockets I really do hope you achieve better results then even you hoped for.

Carl

[Yannis]

Yes, I've read some of the possible negative outcomes from installing pockets but out of my research so many people have talked about the advantages it could offer

I read all the above threads that Tony put forward and I didn't get that impression....
Also, out of all those many people who talked so positively about it, how many of them did this on a 31?

But obviously, as Carl says, whatever floats your boat and we will all be happy to see you satisfied with your choice.
Good luck.

[mike ohlstein]

In rough weather, your pocketed props will be spending a load of time out of the water.

Fine for a flat calm day, but I wouldn't do it.

[andresvarro]

In rough weather, your pocketed props will be spending a load of time out of the water.

Fine for a flat calm day, but I wouldn't do it.

Talking with you guys I am second guessing its application in a 31. Spoke with a few guys in FLIBS '17 including some reps from Cummins who recommended the pockets. So many boats nowadays use them including smaller boats in the 30-40ft range - makes me wonder what makes the 31 hull different and susceptible to those negative performance features.

If only Cabrera spoke good english I'd be able to ask him his thoughts on your points. I guess he would have come up with this design through his own experiences of installing in many boats and tweaking/perfecting over the years.

I agree its a major mod to a famously good hull and wouldn't want to change it if it wasn't for the better.
I'll keep you updated on the build and what I decide on.
Thanks for the input.

[Tony Meola]

When reading through the threads, you will see that at most the gain was maybe 2 knots. Bob Lilco probably did the same if not better because the props are always in clean water. Also Prop Pockets change the back down ability of the boat actually any boat.

What Bob did was drop the engines oil pan are probably at most 2 inches off the bottom of hull. This created a shallower shaft angle without changing the distance of the props from the hull. So basically think of the struts as the fulcrum point. Now pivot the forward section of the shaft down then prop angle changes so the props have started to level out but not there. This allows you to continue to turn a wheel up to 21 inches in diameter without worrying about buring the hull do to prop wash.

This means new shafts and Struts. So if you are going through this process already, it iw worth while increasing the shaft size. Going to 1 1/2 will allow to put in Horsepower up to 330 and if you go up to 1 3/4 shaft you can probably go up to 380 HP which I believe is what Jimmy G has in his boat.

The guy to talk to if that is your desire is Jimmy. I am pretty sure he has the 380 QSB's, and let me say that boat gets up and goes. He told me it is stable at top end which to me means he changed the shaft angle, but not sure. You would need to talk to him. I am pretty sure he is topping out at close to 40 knots.

I don't remember the HP on Bob Lilco's but for some reason I am thinking 370. I have never ridden on his boat but he swears 36 knots is no problem for him.

So there is a cheaper and maybe better way of building the mouse trap.

The other issue with prop pockets, is you really need to make sure you are not compromising the hull integrity. Remember you have basically rebuild that stern hull area along with the transom.

I ahve never talked with anyone who has put in pockets so I can not say how it will impact performance other than what I have read.

Also remember you asked what makes this hull different, I think on modern boats, you don't find too many pockets on a 22 to 24 degree V hull. That might be the key.

What ever you decide best of luck.

[Tony Meola]

One other thing, we are not being critical of what you want to do. We are just trying to offer some advice from what we know about how these boats perform.

In fact if you do it, we really want to know how you make out. Maybe you will give others a path to follow.

[andresvarro]

Every little bit counts where I can gain some extra speed and efficiency so even if it were 2 knots extra I'd be okay with it. The cummins are some solid engines and although I may eventually repower to a bigger powerplant I have a feeling this pair will last for years to come so I've got to make it work although I'd love some extra power.

Perhaps a combo of the tunnels and Bob Licos setup of reduced shaft angle through lowering the engine will create a winning setup for a bit more than the claimed 2 knot speed increase.

If anything, the tunnels should beef up the structural integrity at the stern. They are reinforced and integrated into the hull creating some added rigidity. Unsure as to how they would negatively affect the back down ability - thought it would actually help with this as there is less resistance from a big flat transom pushing the water?

Most people have preconceived notions of what will happen through their experience of owning one over the years but nobody has laid out a solid argument with hard evidence to back it up with from the threads I have read. I think if you have a negative outlook from the get go you'd have negative results. Gotta think positive!

If tunnels don't work on boats from 22-24 degree deadrises, don't know why Bertram would go ahead and incorporate them into their new 35 which has a 22 degree deadrise.

I'm actually enjoying the critical arguments you guys are offering. Its evoking some technical thinking so no worries.

[andresvarro]

Bertram 35

[Tony Meola]

Andresvarro

I understand. But take a look at the hull you posted in you pic and take a look at yours. Two completely different designs. So the tunnels will have a different effect you this one than it will on our 31's.

Keep us posted, we really look forward to hearing how they work out for you. We all hope they do what you expect them to do for you.

[Carl]

I believe the main benefit of prop pockets is that they reduce the shaft angle.

Less shaft angle is more efficient... less down angle is also not lifting stern up/ forcing bow down at higher speeds.

Three ways of skinning that cat...lower motors, moving struts back further and prop pockets.
Or any combination...
Bob went with lowing motors and a ton of other tricks like weight distribution.
Elite Marine has played with moving struts back, longer shafts and placing rudders off the stern.
and you know about pockets.

I think the issue with gaining speed with pockets is while you gain advantage reducing shaft angle your also losing having the props in clean water flow.

Pockets also reduce the ability to walk the stern...at least in the few boats I have run with pockets. Not an end all.

Major mods all seem to enjoy the benefit of larger rudders...



Not having been on a 31 with pockets, I could not say anything for sure. But what I have seen is a fair amount of 31's For Sale having Prop pockets, rather high hp motors and performance numbers that are lackluster AND as they are ADVERTISEMENT LISTINGs they need to be taken with a grain of salt.

As you said...have to be optimistic in your point of view...but as the pockets have been around for years and I know lots of 31 major mods...not too many with them in comparison and have not seen or heard of one that I can say...wow...expensive, but look at those numbers.

Maybe I'm just in the wrong crowds.

[Rawleigh]

I would ask Cabrera for a reference to a 31 running their pockets that you can take for a test run. That might help you have enough info to make a good decision. Rawleigh

[Yannis]

Rawleigh,

I wonder if I should modify my car into a Formula one!

[Tony Meola]

Rawleigh,

I wonder if I should modify my car into a Formula one!

Yanis

Only if you are related to Mario Andretti.

[Yannis]

Tony,

I only thought Rawleigh would propose to talk to someone, so that I could make a round in a F1 car sometime soon!!!

Andresvarro,

What's with this 2 knot additional speed thing?
Somebody here said that the point is not only be fast, but also (mainly) it's a matter of the sea conditions...
My boat goes up to 31 knots, but in a heavy chop if you go anywhere above 25 (or even less) you're airborne, and by the time you realize it you're in for new shafts...if you can seal the shaft holes and not drown, that is.
A boat is designed for a specific purpose, maybe, I'm just saying maybe, a cigarette type boat is more suitable for what you may be looking for.
Also, as I read through this thread, it appears that with your hp you will not see any benefits from the pockets. People here suggest that you need a lot of power to reap any benefit.
You might have to check if even the 315's may be considered "enough power" for this exercise.

[Pete Fallon]

Andresvero,
First off the prop pockets in a 31 is a waste of time and money. You have 210 hp engines which are very fuel stingy, but the 310 to 315 hp range is where a 31 runs the best. I had a 1961 31 Express that came from the factory with direct drive 413 Chryslers. After repowering with 440 Chryslers in 1979 I didn't gain anything but burning more fuel. In 1997 I bought a new pair of 454 &.4 L throttle bodied fuel injected Mercruiser detuned to 310 hp. I also changed the trannies to 8 degree down angle Hurth !:%:! ratio. The boat originally had low profile struts with intermediate strut bearings and 3 blade bronze 16x11 props. The low profile struts allows for a flat angle ( Main struts were only 9-1/2" from the bottom of the boat to the center of the shaft barrel which only allowed a 17'x19P 3 blade prop in order to leave 1-1/2 " of clearance between the tip of the prop and the hull bottom, any closer you end up with cavitation burns in the fiberglass. The shafts were 1-1/4" Monel at 105" long from the coupler to the end he shaft. the intermediate struts were only 4" from the hull to center of the barrel 6" from the shaft alley exit.
After installing the new engines and checking for proper alignment I took it for a test run in lAKE wORTH between Old Port Cove and the bLUE Heron Bridge. There were 3 people on board, 170 gallons fuel 25 gallons water and the soft top and 21' Rupp Outriggers up, 5" exhaust with mufflers.. The wing was blowing 20 out of the SW and a 2 ' chop.
The North Star 951x GPS read 41.8 MPH in both directions at 4350 RPMs. Burning 22 GPH each side. Originally with the old set up I could not get the boat to go over 30 MPH and she burned 60 gllons an hour at 4400.
So change the shaft angle and you get real good numbers. the flatter the shaft angle the better she ran, without adding prop pockets. I could go on for ever about that boat and the way I had it set up and when I get my next 31 she will be set up very similar, and it will be a gasser I don't like the smell of diesel over the transom.
Pete Fallon

[Carl]

What's with this 2 knot additional speed thing?

Somebody here said that the point is not only be fast, but also (mainly) it's a matter of the sea conditions...
My boat goes up to 31 knots, but in a heavy chop if you go anywhere above 25 (or even less) you're airborne,

Yannis-

Your too pragmatic...

1st, a 2 knot gain is big and a worthy endeavor for many. Even IF you do not use the speed it usually translates into better efficiency for reduced cost or more travel time per tank.

2nd...if you can't gain 2 knots, the chances of gaining 3 or 4 more are reduced.
Boats are a luxury fueled by desire; once you attain one the search for "more" sets in...bigger, better, faster.


More speed...even if you cannot use the speed, it's nice to know its there. Might be more a States thing...more power under the hood means you can go slower because you and everybody else knows you can go faster.


My hesitance on pockets stems from there seems to be better ways to achieve the performance numbers.

Then again, I frowned upon outboards on 31's. But a couple have made a good case for going that route, they have good performance to back it up as well, and it fits their use. So while not for me, I do see they can be a viable option to some. So who knows...

[Yannis]

Carl,

Yeah, sure, all this is true, I tried the logical path; you’re on the sentimental one...it’s all ok with me.

The point remains though, how fast you can really go in everyday conditions, before all pans and pots are on the floor, your crew swears it’s their last trip out with you and you’re in an ambulance on your way for an MRI on the 4th and 5th vertebrae!

Perhaps it’s the kind of chop we have here which is very rough at times, most of the times, in the summer. We may not have those huge, far apart waves of the ocean, but the powerful summer winds usually dictate to you to do one of two things: either take it easy with your friends on the dock and continue counting beers and ouzos, or, play it Captain Nemo whereby by the time you reach your destination, usually 1-3 hours away, you’re exhausted and you have to count on luck for what you’ll discover on the decks or under the decks, when you finally step down from that flybridge. That’s why sailboats are sooo popular in the med, because they are the least harmed (if not benefited) by those winds.

Now for the calm days, yes, I agree, what I do is I step on my buddy’s 3X300 outboard CC and fly. I'd have to make a huge investment to turn a Bertram into not even as close a speed daemon, as that cc is. For 10 days in a year...

[SteveM]

I have not ridden in a prop pocketted 31.
My first guess is that you might lose some control with the rudders and wheels not as far below the boat.
Sort of like a Jet Boat may feel.

Shallower draft is a plus to the pockets.

I'm an advocate of leaving someone else's engineering alone, as I am not an engineer nor architect.

[Carl]

I have not ridden in a prop pocketted 31.
My first guess is that you might lose some control with the rudders and wheels not as far below the boat.
Sort of like a Jet Boat may feel.

Shallower draft is a plus to the pockets.

I'm an advocate of leaving someone else's engineering alone, as I am not an engineer nor architect.

I used to have an old Penn Yan tunnel drive boat...ultimate deep tunnel hull had wheel completely in there. Little bit of the strut protruded below hull as did some of the rudder.

Backing up...it went pretty much straight back no matter how wheel was turned. Going forward it handled just like any other boat. Never had issue with it loosing a bite on the water and I was out in some pretty good snot with that boat. But its biggest and only downfall is that tunnel ate power...period. Run the numbers, it never lived up to them. Very similar inboards running the same 318 Chyrsler ran faster with better burn.

[bob lico]

STOP------STOP------STOP there is positively no 2 knot gain with prop pockets and 210 hp Cummins. i not even going to explain this stupidity.one more time the gain of the reduced shaft angle is equated by the meager hp 210 hp Cummins in totally AGITATED water in the pocket you need 380 hp and type Veer 5 blade props to take advantage when the boat rises out of the water you will gain by less drag BUT YOU NEED HP!!!!!!! (tony i have 330 hp Cummins. engine height is preset by the end of gears being 1/2" off bilge, cannot set engines any lower, never measure at the oil pan but no more than a couple inches. engine, gears, shaft and loose struct all install one piece with kevlar shaft log that was able to slide fore and aft with a large slot cut in the bottom to accept shaft log when done with alignment.
.

[Marlin]

I think there is considerable science behind the tunnel concept. My understanding is the Boat creates a shallower draft because the running gear is raised up into the pockets. I have a 42’ margaritavich that was( desined for jimmy Buffett to cruise shallow waters in the Bahamas )which has gone thru numerous prop sizes and several rudder changes. The hull was designed with this feature by a naval architect firm Mel Blount out of the Carolinas, we finally got less vibration in the cockpit by going with 5 blade custom wheels, a clearance of 2” between the hull bottom and wheel tip( cavitation) . I’m currently running the 4 blades as the bottom jumped up at me in low tide recently and there is is a considerable comfort level difference however she is 3 knots faster with the 4 blades and a better fuel burn rate. When I redid my 31 , I ask Cummins about installing the pod drives and their wisdom was, the hull needed to be designed for this type propulsion, u can’t build it in now,FYI

[Kevin]

I took many pointers from reading Bob's posts over the years. Two knots is a big deal and attainable without much work.
Read through all his posts about weight distro, propping etc. I picked up right around two knots with his advice. 25 knots all day long at 14 to 15 total gallons an hour. It's a twenty eight Bertram not a 31 but all the same principals apply.

[Tony Meola]

STOP------STOP------STOP there is positively no 2 knot gain with prop pockets and 210 hp Cummins. i not even going to explain this stupidity.one more time the gain of the reduced shaft angle is equated by the meager hp 210 hp Cummins in totally AGITATED water in the pocket you need 380 hp and type Veer 5 blade props to take advantage when the boat rises out of the water you will gain by less drag BUT YOU NEED HP!!!!!!! (tony i have 330 hp Cummins. engine height is preset by the end of gears being 1/2" off bilge, cannot set engines any lower, never measure at the oil pan but no more than a couple inches. engine, gears, shaft and loose struct all install one piece with kevlar shaft log that was able to slide fore and aft with a large slot cut in the bottom to accept shaft log when done with alignment.
.

Bob

The idea is that to do the pockets now with the hope of upgrading the engines later. So he will not realize much gain now it would be down the road.

Yannis

To answer your question on speed vs size of the boat, I will go back to what I have said before, when I repowered and was thinking about the 315 Yanmars or Cummins, the diesel guy said to me: "Tony, it's a 30 foot boat. You are not going to be able to use all that extra speed on most days offshore. Save some money and work, put in the 270's because they have fresh water cooled turbo's and you only loose a couple of knots which you probably will hardly use."

The I thought back to one of JP's posts about making a bunch of Canyon runs and only being able to average 14 knots because of the weather.

One just needs to remember that famous quote we always say on this board,

"The enemy of good is better."

[Yannis]

Kevin,

What did you do? You didn't lower the engines or any such major task, right?
Reason I'm asking is that I haven't touched anything on my boat and equally do 24 knots all day at 55-60 liters an hour.
I think you have smaller engines though, so you must have done something!

Tony,

That's exactly my point too.

[bob lico]

Marlin, you got it" custom 5 blade props " as in veers. when you think back to a tunnel hydroplane that can doo close to 200mph and is literally riding on the prop. we understand with no drag can do.However, prop pockets on a 31 Bertram is not a catamaran nor is it a tunnel hull!!!!! you could build from scratch and put two air steps from each side to allow air to come in and help reduce the drag. you also forgot to mention the deep vee in combination with huge rudders save my life every damn time i have run out Fire Island Inlet in small craft warnings, the boat can turn 45 degrees in a second on a cresting 10' wave.
the subject of speed yannis i always run her at 30mph offshore only when there is those constant 4' plus waves i slow to 25 that is her sweet spot at that speed if you sat in the cabin you can really hear the loud slapping sound as the crest of the waves hit the bottom of the boat you also cannot "just back off" you will slam that bow into next wave. we use 315 hp ALL time offshore even to shipwrecks 40 miles or less offshore for bottom fishing.Phoenix has NOAA Headboat status and must report catch every month. think of it as a modern performance car. you can do 80mph at 1800 rpm in Corvette or you can run the balls off a 4 cylinder and do 80 mph the one-time fuel economy advantage is lost in that 4cyl. engines because you're increasing the RPM.. i run my Cummins at 2350 rpm. to maintain my cruise speed with these props i designed. the wot. is cut to 33 knots but cruise speed is super fuel economy. 4cyl, Yanmar at 240 hp and my Cummins have the same fuel economy at about 1.8---1.9 miles to the gallon only PHOENIX is running about 5 knots faster at cruise. there is quite a bit of work involved attaining this including getting every bit of efficiency out of engines (like 0 backpressure and tuned 6" exhaust from sprinkler to exit) on engines and boat modifications like streamlines 6" shaft log protrusion as well as modified streamlined structs.fortunately, i had a full facility boatyard with forklift and travel lift that i could use anytime i wanted.(20 prop chances).

the new 35" Bertram was built initially to compete in the day boat market along with Hickly (sp?). she was built in Maine as such and would be built to be used in places like Newport, Martha vineyard, etc. the prop pockets are needed to reduced damage from accidental grounding, prop. protection is very important that is why other builders use the jet drive for picnic boat. later on after selling a few boats they went to a bridge and started appealing to the general public but still have the prop pockets BUT a huge torque catapillar 3126 at 450 hp , remember what i said about hp. and prop pockets you need "balls" to use the pocket for less drag coefficient.

[Yannis]

Bob, thanks.

When you have time, please try to convert all the figures above into ONE unit, that is, nautical miles (for the consumption figures) and knots (for the speed).
There is a significant difference between the two and when you mix them in the same sentence I find it hard to understand the resulting performances.
Unless whenever you say miles, you always mean nautical miles (nm, for short), in which case please just confirm just that.
Thank you.

[Kevin]

Yannis,
Removed antifouling paint and replaced with VC performance epoxy. Store it on elevator lift.
Rounded leading and trailing edges of rudders. They were squared at first.
Turbos are up against back of engine compartment so aft most engine install. This was not by design but because I did not want to buy new shafts when switching from velver drives to ZF. Batteries are mounted far aft in engine space.
Got rid of all the weight that was not needed. Some call it ginger bread. You would be shocked how much crap you haul around that most people never use.
I'm running 5 inch exhaust starting just below the mixing elbow all the way out.
Props......I spent alot of time experimenting. More cup is better than more pitch. You lose top end max speed but gain it at cruise. On a lift I just use skiff for prop swaps so no boat yard expense.
I have 212 hp and reach manufacturer specified rpm at WOT.

[bob lico]

yannis we cruise at 28 knots at 1.8 to 1.9 statute miles per gallon of 45 cetane diesel fuel in 4' or less wave height and drop down to 24 knots in bad weather crossing waves and waves beyond 4' in height unless a following sea then back up to normal cruising speed. fancy driving (like a sailboat tacking) and large waves also kill the fuel economy.

i should also mention going back to my original post and for those who want to know the BIG WHY on the prop pockets for 31 Bertram . 31 Bertram has a 24 degree deadrise from bow entry to stern so you want to cut a perfect all grass hull apart keep in mind you will have a 24 degree entry to your prop pocket . perfect if you really wanted to test your skills at building a surface riding prop in the maximum agitated water, this is like the white water in the back of an outboard prop.Incidentally never let anyone hoodwink you into ever believing that molded insert prop pocket is as strong as the original unmolested 31 hull. BS!!!!!

[Yannis]

Thank you Bob. So its about 2.4 liters per nm. This is how boat consumption makes sense to us!

Kevin,

Youre giving me ideas!
I dont know about the antifouling because I stay in the water for 4 full months and I have no idea how this vc epoxy behaves without a lift (btw a lift is a completely unknown notion here,I doubt if even a single boat owner has ever heard about it, let alone seen one).
Also, us who live on the boat for so long, theres only so much...ginger bread we can throw out. We have to have all tools, all spares plus all clothes and all kitchenware, a lot of stuff... plus a lot of water. I have 350 liters of it and only this year I installed a second tap in the head so that all initial rinses as well as all bathroom cleaning is done with sea water. Then we will rinse with fresh water. I expect a lot of water economy from just that.
The rest I have very similar to your configuration, only I carry 4 batteries plus the solar panels...the best improvement by far, as this allows me to be completely independent from any power source.
The props is something I havent even discussed within myself yet, I am a complete ignorant on the issue. I hope they are good!
Good luck.

[bob lico]

kevin you learn well. i give you a "A" for excellence --------seriously fuel is expensive why waste it.