How Much pitch to add

STraenkle · Post by **STraenkle** » Sep 3rd, '09, 10:12

As some might know I recently put new gas engines in to my â€™75 31 FBC. They are Mag HO 496 MPI which put out about 425 HP and they have a pretty flat torque curve of about 450 ft-lbs. Now I have 20 x 20 slight cup on them now and the boat is way under propped. I should max out between 4,400 and 4,800 RPM however I am at 60% throttle at 5,000 RPM so I canâ€™t tell you what the RPM would be at 100%, but from the data it looks like it would turn about 6,000 RPM. The question is how much pitch/cup to lower the Max RPM by 1,500 or should I go with a larger wheel and keep them square, like a 22 x 22.

Are there general â€œrules of thumbâ€ for added pitch and added size to change the WOT RPM?

Also, I have 1 3/8 shafts, how close am I to requiring 1 1/2 shafts to handle this kind of torque?

Thanks

AndreF · Post by **AndreF** » Sep 3rd, '09, 11:07

"They" say 2 inches = 200 RPM, I've always heard.

Carl · Post by **Carl** » Sep 3rd, '09, 11:27

You will loose or gain aprox 200rpm per inch on your pitch or diameter, aprox 200 rpm to go up a cup size. These are Ballpark figures. You have better efficiency with larger diameter wheels until you hit a certain speed then the extra resistance of a larger wheel in the water comes into play.

Aside from pitch, diameter and cup you can also adjust the rake, looks like you have ample hp to let that come into play.

Remember if you "just" increase the pitch you will be traveling a whole lot faster at idle.

My guess would be go to a larger wheel, try to keep square and add a bit of cup to keep the speed down at low speed, yet give the extra boost at higher speeds.

Exactly how much...I would defer to a "Good Prop Shop" you have lots of variables and your making a pretty large change. I can put you in touch with a few if you do not have one. J&B Prop in Jersey, Lawrence Johnson in New York to name a couple and I'm sure several others will chime in. But make sure it's a good knowledgeable shop and not just a guy that bangs em back into shape and sends you on our way.

mike ohlstein · Post by **mike ohlstein** » Sep 3rd, '09, 11:56

You're going to have trouble fitting anything larger than 21" diameter.

Perhaps you should be thinking about a different gear ratio.......

STraenkle · Post by **STraenkle** » Sep 3rd, '09, 12:00

Thanks for the responses.

I am not too worried about the idle speed, right now with both engines in gear I am running around 3 knots. I actually have to throttle up to get the boat into the slip if there is any wind. It is actually hard to tell when the boat is in or out of gear. I think I will have to go to 22 inch wheels and 22 inch pitch to knock of approximatly 800 RPM, from there I think I will add cup to get down to below 4,800.

Anybody need a new set of 20 x 20 wheels? Oh well, I think I will run the season on these and get bigger ones next year. I know I am getting to the largest diameter wheel that will fit, but are many of you diesel guys running 22 inch diameter or larger? I just want to make sure they will fit, looks like 1 1/2 shafts are going to be needed also.

Carl · Post by **Carl** » Sep 3rd, '09, 13:41

Scot what is your gear ratio?

Anything more then 1.2: 1 and your under the Recommended Safety factor of 5 using 1-3/8" A-22.

If you have 1.5:1 your running with a Factor of 4.1

My distributor recommends the following minimum safety factors:

Commercial (10)
Recreational (5)
Sportfish (7.5)

That said I know some guys run well under the recommended factors for years...

Can you borrow a set of wheels instead of buying new or re-pitching? Some shops will lend you a pair to try out.
Running 3 or 4 blade?

STraenkle · Post by **STraenkle** » Sep 3rd, '09, 13:55

I was going to go to 1 1/2 shafts this winter anyways, I just needed to get the boat wet. I am running 2:1 gears, thus the problem with needing larger wheels. The wheels I have now are 3 bladed. Your right, it would be a good idea to go to 4 bladed to get the RPMs down. Does anyone have a good idea of what that might drop the RPMs?

Also, I don't know of a good prop shop to talk to. There is a guy that does a great job at fixing them, but he has no idea about props swaps with wheels this small. He mainly does the fishing fleet props in New Bedford MA. That is why I am asking the guys here who know what prop size changes do to a Bertram 31. Any suggestions for a good place in SE Mass?

scot · Post by **scot** » Sep 3rd, '09, 14:08

Coastal Propeller in Bridge City, TX. 409.735.2488 ask for Mike, he is very good and helpful as well.

I think he is going to want to know "just how high rpms will they go on the pin?"...which is my first question. The 60% reference does not help.

If you have no concern of idle speed, just add pitch and don't worry about all the expense of adding diameter.

Carl · Post by **Carl** » Sep 3rd, '09, 15:33

J & B Propeller in Waretown NJ
732.269.2929
Ask for Jim, tell him what you have and let him give do his thing. He is best at dialing you in and getting the most out of a prop. You may need to have more info then you have now. His work is fantastic...although his 1-2 weeks can be quite a bit longer

Lawrence Johnson Machine Works NY
800.836.2293
Ask for Larry Sr, Excellent person to deal with, should get you in the ballpark without a problem. Lots of experience.

S&S Propeller NY
718.359.3393
Ask for Cathy

They have a great piece of software to play with the numbers in no time, gear ratios, pitch diameter etc. Just watch they don't just try to sell you what they have in stock.

You can tell any of them your a friend of Carls at Staten Island Machine and need advice on wheels.

Personally I'd run it by several shops and see where they come in.

Scots right, you may need to know what your WOT is to fine tune, which means there may be some trial and error involved. I'd opt not to buy new wheels right off the bat. Borrow a set, swap what you have for something closer or re-pitch what you have to get in the ballpark.

My .02

Bruce · Post by **Bruce** » Sep 3rd, '09, 16:01

This is why before the engines and gears are bought, prop calculations are made.

Those engines should have 1.5:1 gears.

You can do calculations without WOT and be very close with just a tweaking left.

I doubt your going to be able to fit a proper wheel with that setup.

If your installer made the 2.0:1 choice, make em change it out.

JP Dalik · Post by **JP Dalik** » Sep 3rd, '09, 16:51

Scott,

I think your guy did the right thing. The only way to take advantage of the torque and horsepower was to maximize the gears ability. You've got plenty of torque so you'll need to swing a larger wheel at 2:1. At 22"s diameter how much tip cleanrance will you have"? You may need to go with a radical styled wheel to maximize efficiency but I wouldn't be afraid of that.

Hell just look at the rake and pitch Lico is swinging.

20x 20 1 3/8" sounds like every Yanmar 240 owners wet dream wheel to buy. Just re-pitch and off you go.

I can second J&B in Waretown (I have no idea where your at) but they've taken care of our wheels both 3 and 4 blade since we've had the boat and all is good. Delivered every Thursday to the Marina like clockwork.

Good luck, I'd love to hear how she turns out.

Bruce · Post by **Bruce** » Sep 3rd, '09, 20:30

The only way to take advantage of the torque and horsepower was to maximize the gears ability.

To a point and then it becomes an obsession to fit 10 lbs of crap in a 5 lb bag.

What I'm curious about is did anyone do any number crunching on the setup before it was installed. I'd like to know what someone had on their mind at the time.

You just don't install a set of engines with the theory of "we'll work it out when we're done".

mike ohlstein · Post by **mike ohlstein** » Sep 4th, '09, 06:41

22" won't safely fit. You will wind up with about 1/2" clear of the bottom.

Sell your gears and buy the right ones.

STraenkle · Post by **STraenkle** » Sep 4th, '09, 07:21

Thank you for the help, I will get 4 bladed 20 inch with 22 pitch and cup it to come as close as possible. Unfortunately changing out the gears is not an option. To do that, I would have to sell the boat. Unlike many here I can not afford it. The divorce, raising three teenage daughters, saving for the college, etc. are much more important than going 40 knots in the boat. I am lucky to even have a boat.

I ended up with the 2:1 gears because of price. Running 250 hour Mag HO 496â€™s with gears for $16,000 from a diesel conversion by eddy at Johnsonâ€™s boatworks. Was a deal that was the only real option. I would have spent the same amount for 454â€™s and no gears and still using some 35 year old parts, and I like the MPI engines. I now get 1.2 miles/gal at 25 knots (30% throttle) and without over revving it, 35 knot top speed. These engines are so lightly loaded with the set up now and I only really want to go 25 knots anyways, I donâ€™t care about the top end, itâ€™s just a whoohoo thing every once in awhile. My only desire for getting the new props is to see if the 25 knot cruise can yield better fuel economy.

As for finding the WOT RPM, that is not possible, the rev limiters kick in way before that. However, from graphing the RPM vs. throttle position, 6000 is pretty darn close. I will talk to the guys you suggested and hopefully get trade in or some money for the props I have now. They too were a deal on eBay that was close enough to go and enjoy the boat.

Thanks all once again and hopefully we can all enjoy the boats this weekend; it is going to be beautiful here in the Northeast. Only problem is ever yahoo that owns a boat will be out, guess Iâ€™ll have to head off shore.

Carl · Post by **Carl** » Sep 4th, '09, 07:59

STraenkle wrote: I will get 4 bladed 20 inch with 22 pitch and cup it to come as close as possible. Unfortunately changing out the gears is not an option. To do that, I would have to sell the boat. Unlike many here I can not afford it. The divorce, raising three teenage daughters, saving for the college, etc. are much more important than going 40 knots in the boat. I am lucky to even have a boat.

Scott, it's all a trade off. We start at 100% and whittle away till we find the option that works best.

I'd still run it past a prop guy...it cost nothing to ask. Some may even have used wheels or tweak yours for about what you may pay for a pair on E-Bay.

lobsta1 · Post by **lobsta1** » Sep 4th, '09, 11:15

Scott,
I just sent you a PM. I may have some props you can borrow to try & get dialed in.
Al

Bruce · Post by **Bruce** » Sep 4th, '09, 13:13

Basic prop dimenson would be 20x25 3 blade and a 20x24 4 blade both with a #4 cup to start.

Wouldn't run them on a 1 3/8 shaft though.

With that much pitch watch for the ass end climbing up and the bow nosing down.

scot · Post by **scot** » Sep 4th, '09, 15:28

Unlike many here I can not afford it. The divorce, raising three teenage daughters, saving for the college, etc. are much more important than going 40 knots in the boat. I am lucky to even have a boat.
I ended up with the 2:1 gears because of price.

Scott I hear ya. Price is always high on my list as well, and you did get a swingin deal. The "perfect" set up has never been in my budget, so I have to make things work, with what I have.

Just add 4" of pitch + cup as Bruce suggested and see what happens, it sounds like your are REALLY under propped and I can't image the 4" adder dropping you below 4200 rpms based on what you have stated. Unlike Bruce I think the 1 3/8" shafts will be fine, assuming all else is in order and properly aligned.

BTW my last daughter is in her LAST year of college!!! Major raise coming in 6 months. Girls are great but they can go through money like a corn through a goose.

Bruce · Post by **Bruce** » Sep 4th, '09, 17:01

Unlike Bruce I think the 1 3/8" shafts will be fine, assuming all else is in order and properly aligned.

Well since my advice is not based on alignment and everything being in order but on torque loading on the shafts, I'd say my advice was sound.

You guys gotta keep in mind that for the few regulars we see, there are many members who don't post or ask questions or even those who visit regularly and don't register that look to this web site for sound advice and not shade tree or by the seat of the pants mechanics.

Pat and I get calls all the time from these folks who get confused over the sometimes wide variety of advice.

Everyone here has a valid opinion but what everyone does with their own boat is okay, but when dispensing advice please keep this in mind.

scot · Post by **scot** » Sep 4th, '09, 19:03

Bruce,

I'd say my advice was sound.

I agree, I think your advice is sound, but I also believe mine is as well. Your advice is typically based on "best practice" (always a good thing to follow). My advice on the other hand is commonly based on my personal experiences, research and financial "get-by-with".

In a recent trip to the local prop shop I was pricing and discussing various wheel sizes for my project and learned the following: The Michigan Dyna-jet (3 blade) or Dyna-Quad (4 blade) in 20"D x 24"P is avialiable from Michigan in the following bore options: 1 1/4", 1 3/8", or 1 1/2". So this is what I was basing my opinion on. Not exactly shade tree or by the seat of the pants mechanics, but deductive reasoning. But I do work on stuff under a shade tree so I qualify for the title.

But I will concede and freely admit that your experience and knowledge base far exceeds mine. In the future I will add IMHO (in my humble opinion)

DRIFTER31 · Post by **DRIFTER31** » Sep 4th, '09, 19:25

One thing to keep in mind for sure with small shafts is to "GENTLY" apply throttles when getting on plain etc... or you may snap one. Also i would buy 1-1/2 bore props and use the bushings till you have the $$ to put the larger shafts in. Good luck Troy
keep us posted on the results and gas consumption

bob lico · Post by **bob lico** » Sep 5th, '09, 17:37

i must say one thing scot is on the ball with the dyna-jet as a start in 21"the only thing i have to add and this will get you in trouble at a ordinary prop shop.you want a radical cup ! start from the hub and go all the way around the tip and half way down back side.yep you heard me correctly you will add increase effiency. you will also need about 2 degrees of rake to compensate for the shaft angle ,just like a outboard or sterndrive you wouldn`t expect to cruise with the drive tuck in would you! then why would you do it on a straight inboard. like bruce said go 11/2' but for now use 11/2" to 13/8" bronze bushings.i like pitch at no more then 24" trust me that cup will cut that rpm dramatic.boat with be bow proud at all speeds .sim mention s&s they do this work.

Bruce · Post by **Bruce** » Sep 5th, '09, 19:05

Scot,

The mechanical descriptions were a generalization and not personal one toward you.

If I had meant it to be specific toward an individual I would have said so.

Like I said what everyone does with their own boats is fine.

Bob,
I'm sure you got the in business discount but if you don't mind me asking what was the complete costs for starting out with prop experimentation to the final product including the prop cost itself?

I'm trying to do a cost comparison chart vs performance.

bob lico · Post by **bob lico** » Sep 5th, '09, 21:37

the owner phil did them himself after detailed desigh inputs from both of us. BOSSLER&SWEEZEY 285-0458 $292.00. phil has been in the business 45 years ! a real gentlemen.i can`t describe the effect of that wild cup .every bit of water is use to push foward maximum effiency.

JohnCranston · Post by **JohnCranston** » Sep 7th, '09, 20:49

Scott,
Give me a call if you decide to unload the props.
Thanks.
713-906-0842

Carl · Post by **Carl** » Sep 8th, '09, 12:29

scot wrote:Bruce,
The Michigan Dyna-jet (3 blade) or Dyna-Quad (4 blade) in 20"D x 24"P is avialiable from Michigan in the following bore options: 1 1/4", 1 3/8", or 1 1/2". So this is what I was basing my opinion on. Not exactly shade tree or by the seat of the pants mechanics, but deductive reasoning. But I do work on stuff under a shade tree so I qualify for the title.

When you do the calculations for a shafts safety factor, it's based on the Alloy Grade - HP - Engine Speed - Ratio and Shaft Diameter. Nothing about the wheels.

Aqualoy recommends being over 5 for recreationsl boating 7.5 for a Sportfish application, plug in Scots numbers and you come up quite a bit short.

Scott has alot of available hp, with the small wheel and pitch he's not using that hp...once he starts adding pitch / diameter / cup he's going to be using more of that HP and reducing his safety factor. Does that mean the shaft is going to snap...no but the chances of it snapping go up quite abit.

As far as the wheel being available in those bores...alot has to do with the application.

STraenkle · Post by **STraenkle** » Sep 8th, '09, 20:12

To all, I do appreciate the advice and help, I knew going into this that the boat would be way under propped (obviously better than over propped), but I like the way it is, itâ€™s faster, better MPG and reliable. I knew that if I got these motors I would have to up the shaft size, if and only if I choose to â€œhit the throttleâ€ I know that I will up the shaft size over the winter, thus the questions about upping the size from the experts here on this site. It has been my experience that everyone has opinions, and some are not worth listening to, but many are and once again sort through it and make your decisions. I will go to 1 Â½, I will up the pitch and live with thegear ratio. I will see if I can get higher rake and four blades, but at the end of the day, I have very reliable engines, 20 percent better MPG, and a grin from ear to ear that the boat runs so well. And for 16,000 dollars and a lot of sweat equity work, the Bertram would make anyone grin as I go by.

STraenkle · Post by **STraenkle** » Sep 8th, '09, 20:16

After reading my last reply, I want everone to know that I did and do appriciate the help and responces from all and not one of them was bad advice, all of it was and is appriciated.

Scott

Bruce · Post by **Bruce** » Sep 9th, '09, 06:01

I'm closing this thread.

ST,
If your happy with what you have, then that's all that matters.

bertram31.com

How Much pitch to add

How Much pitch to add

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