Questions for Gas-powered 31 owners

[trace elements]

All,

Have the opportunity to purchase a 31 FBC with twin gas 454s. Motors are 1996 crusaders.

First, what are realistic performance expectations of a 31 with 454s in terms of speed and mileage? Does anyone have any speed, rpm, mpg info they would be willing to share? I have heard 1mpg thrown around alot, but I have also heard people report 30 gph at 20 mph = .66mpg.

Second, how many gas 31s are left? Doesn't seem like there are too many gas boats left. I'm sure diesel is superior in this application, but I have been presented a fairly unique opportunity to get into a gas boat for a good deal. Hence the question.

I don't do any offshore fishing. Range isn't a concern. Mainly cocktail cruising, bay fishing, and some overnighting.

Thanks!

[mike ohlstein]

Gas is great for Howdy Boating.

I used to get .8 mpg at 18 knots. Better at trolling speed, much worse at ramming speed.

[John F.]

I had 454s in my 1969 FBC. Around .8 mpg or so. I never really checked. Once the secondaries kick in, the mpg plunged. Gassers are quiet, no dismal smell, and as I remember she cruised at around 20 knots at 3000. Make sure the B31 you’re looking at has had the tank replaced. Gassers are great, but the market for old sportys is pretty weak. My current B31 has diesels and cruises real easy at 23-24 knots and burns about half the fuel as my gasser did.

[trace elements]

Thanks, guys. Sounds like 1mpg might even be realistic, but in the ballpark.

Boat has the tank replaced, bu t the deck was replaced with starboard, which I would like to redo.

[Tony Meola]

Do yourself a favor, a good deal is too good to be true. Get a survey before you seal the deal.

[Stephan]

Trace-
There are many of us with gas engines out there. In fact, the RLDT moniker is only half true unless you have a gas boat.
I have a 1972 FBC with 1992 Crusader 454 engines. I think my boat is comparatively light as it floats well above the chines unless loaded with full fuel (205 gal.), ice, etc. My props started life as and are stamped 18X18 however the measure 17.5X17 and I have the Bertram small rudders.
In 2015 I installed Garmin GFS10 fuel flow sensors and an NMEA 2000 network. I have checked the fuel flow/usage numbers against the gas pump and they appear to be accurate. I did replace one of the GFS10s this spring when it failed to wake from its winter hibernation.
The data I have is that the efficiency curve is relatively flat. It is hard for me to do much worse than 0.8 NM/Gal unless I really lean on the engines. From 20 to 24 knots I get +- 1 NM/Gal. (2,600 RPMs to 3,100 RPM) for those speeds. It actually gets worse if I slow to 15 knots falling back down to 0.8 NM/Gal. If I get into the secondaries I get 28 knots at 3,500 RPM and 0.7 NM/Gal.
The boat seems to like 2,600 RPM and 20 Knots which yields 1.0 NM/Gal. This is my customary cruse power setting.
All of these data points are from fair weather. I believe gas engines especially are subject to higher consumption in rough seas and high winds as they build torque at higher RPMs.
If nothing else it will give you a way to make your kids sit still long enough to take their picture:

I get it about diesel engines and would have them if I could. However, these engines have been so good to me and the rumble when they start up and the howl at 3,500 RPM makes me smile every time.
I'm in Rhode Island, if you would like to take my boat for a ride or if I can be of assistance in any way please just let me know.
Regards,
Stephan

[bob lico]

tuesday,August , 25 2020 if anybody can get the EPA, diesel "sniff detector to rise above 0 ,i robert lico will burn my 31 to the water line!!!!!!!!!! the only reason anybody can smell diesel is if the installer is stupid. i have supply photos and explanations for 18 years please refrain from using that sentence "diesel smell"

[Carl]

With my old carbureted 440 gas motors Id cruise about 18mph at 2850rpm and burn .9 gpm. I could ease up a bit and get a little better, push and get a bit worse.

Short trips it's tough to justify diesel...you just don't burn enough fuel. Start taking longer trips more often and things change. If my motors where not 40 years old, 30 years since a rebuild...id have kept, rebuilt and reinstalled.

The boat runs well with gas motors.

[Craig Mac]

I have 454 carb motors---and see similar #'s as Stephan---"From 20 to 24 knots I get +- 1 NM/Gal. (2,600 RPMs to 3,100 RPM) for those speeds."-----
my boat is also light----even with a full tank mine chines are above the water---
I think weight is the key to get good performance from gas motors----the diesels don't seem to be affected as much.

[trace elements]

Some of you guys reporting 24 knots is great. I would be happy at 20 knots but it's nice to have the extra if you want it.

What is everyone's specs?

Motor year
Rated power
trans ratio
propeller type, no. of blades, pitch, diameter?
Anything else of note?
Has anyone tried acme props or 4 blades?

I spent a lot of time extracting all of the performance I could from my Mako and had fun doing it. Looking forward to this opportunity.

Thanks

[Craig Mac]

Again my boat is pretty light---two different travel lifts put it abt 11K lbs--------Borg Warner 1.5:1 reduction---3 blade Michigan Equi-Poise --18 x18--tip cup.

2600 RPM 24 MPH 21 KNT

2800 RPM 26 MPH 22 KNT

3000 RPM 28 MPH 24 KNT

3200 RPM 30 MPH 26 KNT

These #'s indicate a very low slip 17-18%------If you refer to an article posted by Paul Gozewski "CAPY" on the original boards----a slip # like this was what he attributed to the larger

diameter props of diesels ----he thought smaller diameter gas boats would have a slip of 24-30%.

Some of the real early gas boats had shorter struts and were turning 16" diameter wheels with a 1:1 transmission----more efficient shaft angle--but not sure how it affected slip?






I do know someone that put ACMES on this summer---but don't know his before & after #'s ----and I expect him to be heavier--he runs pretty loaded.

[trace elements]

Bob, I’m not sure what you mean with your comment about diesel smell. The only diesel smell I know is the smell of diesel exhaust and most of the diesel boats I’ve been on had some diesel exhaust smell to them. (I worked at Viking for a couple years and even those had a diesel smell sometimes.)

That being said, diesel smell is no reason to discount diesels over gas in this boat. The Diesel engines are definitely better performing, I don’t think anyone is arguing that. Just in my case the price premium for a diesel boat probably isn’t justified for the type of boating I do. I would love to have diesels.

[PeterPalmieri]

454s similar performance numbers to Craig. As long as your not doing long trips they are no problem. That being said if you are looking to buy a boat it’s not that much more to find one with diesels.

[Carl]

I think the smell was in response to a comment I had made about the smell of diesel boats.

I used to really notice the smell of diesel fuel on a boat. In the engine compartment, bilge, sometimes where the tools were stored and of course when fueling up. A faint smell of the exhaust on the cloth in the cabin...
After bringing down 90 gallons of fuel in jerry jugs, running fuel lines and bleeding the motors I think I desensitized myself. I noticed it when fueling up and thats about it. The exhaust that on the first start up I thought I made a mistake converting...I hardly notice it anymore.


As to justifying...its a boat. Unless your fishing to eat or it's a commercial boat there is little reason to support buying it. A boat is for enjoyment...if you have the means to buy a diesel boat and that will keep a smile on your face, go for it.


You already know you don't NEED diesel motors for the way you intend to use the boat. You already know gas motors could push you along just fine. And you already know you don't care, you want a diesel boat. Maybe you just don't know if your want justifies the expense.

[trace elements]

Carl,

Yes, agree on the smell. In my experience the smell of diesel exhaust I am more sensitive to, however, that would never stop me from buying a diesel boat. In this case, it’s simply the cost delta. The gas motors will be more than sufficient for the boating I do. You’re right, I can’t justify the increased cost and potentially higher maintenance cost of diesels. So has it will be. Thank you all for the input and keep it coming.

[Tony Meola]

Actually that was my wife's fear when I repowered. But for some reason I really do not get any diesel fume smell while running. At idle with the wind on the stern yes we get the smell but actually not too bad. I was really surprised at how little it bothers us.

[Yannis]

Sailboats are almost 100% diesel. In certain occasions, when motoring with the wind, all the fumes go with the boat.
Unbearable smell that nauseates you, not just “bothers’ you. Yet, no sailor ever complains.
Try to cruise in a Blackfin with an isinglass wrap on the bridge...
In my 28 there is no smell, can’t even tell it’s a diesel.

[Amberjack]

Besides, a whiff of diesel exhaust on a cool, crisp morning when untying the lines is one of the finest smells in the world!

[JohnV8r]

I'm in Northern California where the weather offshore is not always as nice as it is on the East Coast or in Southern California.



You can see from the picture I appear to ride high in the water, but I think that has to do with the weight of the bow pulpit, anchor, and windless. I don't think of my boat as lightly loaded due to three battery banks (six Group 31 batteries), corian countertop, 43 gallon baitwell, and my propensity to want to take everything with me. I am as aerodynamic as a brick with the enclosure, but warm & dry. I have 1978 Mercruiser Series 72 330 hp engines with the original Velvet Drives at 1.52:1 and 3-blade 18x19 props. I have a full Stamoid enclosure. Wind is typically 15-20 knots in the afternoon here. Sea state generally keeps my cruise 20-22 knots at 2850 rpms (just below the secondaries) as I would generally rather not spill a drink than get an extra 2-4 nautical miles per hour. I can troll for salmon (2 or so knots) on one engine, but never ever shut the other engine off on a trip.

I have notes on every trip and subsequent fill up since 2003. My albacore trips are my longest seasonal trips and average 162 miles round trip from my trip notes. Every single trip (cruising, trolling, and drifting) I have run since purchasing Shambala in 2003 has averaged somewhere between .88 and .92 mpg.

[trace elements]

She arrived yesterday. I am going to do some maintenance to the engines and will get some updates on performance numbers. I need to figure out how to network fuel flow senders, I am not sure if the current gps is modern enough to connect to nmea 2000 senders.

[PeterPalmieri]

Congrats and best of luck. We bought our gas powered 31 in 2011, at the time it is what we could afford comfortably. It’s been an uphill battle and a ton of work just to keep up with so many 50 year old components and have absolutely loved every minute.

One of our beloved, now deceased members, Walter. Said to me use the boat for a year, don’t change anything and figure out what works for you. For the many of us that haven’t done a stem to stern refit it’s a constant evolution and a real sense of joy. Best of luck.

[trace elements]

I ran the boat again all weekend and she ran great. The tachs are a little wonky and will need to be replaced before I can get accurate speed numbers. At what I approximate to be 3000 rpm I am getting 18 knots and at 3200 I am getting 20 knots. I have no idea if this is accurate or not. I will replace the tachs shortly and advise back. Also, probably looking at having both carbs rebuilt. The starboard motor is hard starting cold and has had the mechanical fuel pump disconnected. It now has a marine electronic fuel pump mounted to the bulkhead feeding the carb fuel. I'm not sure why this may be. When the boat comes out in the fall the motors will most likely get new fuel pumps and rebuilt carbs. Per the advice above, I'm just going to run the boat for now and get my list ready.

[Tony Meola]

Your numbers are close. With the old 454's at 3000 RPM we were at 19 knots.

[trace elements]

So I think the one working tachometer I have been using to gauge my RPMS is reading approximately 500 RPMS high. Using a phototachometer, I am getting 20 knots t 2600 RPM.

[Carl]

Now those numbers do not sound right. Maybe 20 mph, but even that sounds too fast for 2600rpm

[JohnV8r]

Agreed. I am 17-18 knots at 2600 rpm, which calculates to 19.5-20.7 mph.

Interestingly, I have found 2850 rpm is my sweet spot for fuel economy. That usually puts me between 20-22 knots depending on sea state. I have installed the Garmin fuel flow sensors, so I should have even more data sometime later this year.

[trace elements]

I am going to run the boat again this afternoon and check again. Maybe there is an inaccuracy in the photo tach, or the boat is steeply overpropped, it’s hard to nail down one variable sometimes. Thanks for all the input.

[Carl]

If over-propped you will not hit your rated WOT. Make that your 1st concern, then you can play with speed, fuel burn, best cruise etc.

[trace elements]

Wanted to update this thread. I bought the boat last fall and fished her through the fall. The boat had OLD tachs which weren't very reliable. We were cruising around what I thought was 3000 RPMs and 18 knots. Turns out we were probably cruising somewhere around 2400-2500 RPMs. Fast forward to this spring, I completely rewired the boat AC and DC side and fixed a bunch of issues with the engine's harnesses and some other items. I encountered some issues with some of the engine senders not using the new standard resistance ranges, the old VDO instruments used different resistance ranges. So now most of the engine senders are new as well as the harnesses.

Running the boat some of you might have read my other thread about the acceleration issues I have been having. Anyways, from a performance standpoint, the following data is from the dash tachs (photo tach verified at 3000, didn't verify the others), Garmin GFS-10 fuel flow senders, and GPS.

20 knots at 2600 RPMS, 1.1-1.2 MPG

23 knots at 3000 RPMS, 1.0 MPG

27 knots at 3400 RPMS, 0.9 MPG

31 knots at 4200 RPMS, 0.8 MPG

Hope this helps someone in the future. I appreciate all of the help from the group. Plenty more to do.

[JohnV8r]

Thanks for mentioning the sending unit issues. I am going to check the RPM's vs the tach's.

I bought the Garmin GFS-10 fuel flow senders as well. In the final stages of what feels like an eternity out of the water. Cannot wait to see what they show for Shambala. I always thought I was averaging .88 - .92 NMPG cruising at 2850 RPM's based on fuel burn at refill vs mileage on the GPS. Based on your numbers, I'm not so sure now.

Hope to splash by July 4th.

[scenarioL113]

Bob, I’m not sure what you mean with your comment about diesel smell. The only diesel smell I know is the smell of diesel exhaust and most of the diesel boats I’ve been on had some diesel exhaust smell to them. (I worked at Viking for a couple years and even those had a diesel smell sometimes.)

That being said, diesel smell is no reason to discount diesels over gas in this boat. The Diesel engines are definitely better performing, I don’t think anyone is arguing that. Just in my case the price premium for a diesel boat probably isn’t justified for the type of boating I do. I would love to have diesels.

I can tell you that I finally got my props correct and I now have ZERO smell and ZERO smoke when I am powering up to get on plane. I messed with my props for the last 12 yrs and finally got hem right. I can not believe the difference in the exhaust from just propping correctly. I did lose some performance numbers BUT engines are running happy. So it takes me a few more minute to get to where I am going...

Many manufacturers over prop their boats when brand new.

[scenarioL113]

Wanted to update this thread. I bought the boat last fall and fished her through the fall. The boat had OLD tachs which weren't very reliable. We were cruising around what I thought was 3000 RPMs and 18 knots. Turns out we were probably cruising somewhere around 2400-2500 RPMs. Fast forward to this spring, I completely rewired the boat AC and DC side and fixed a bunch of issues with the engine's harnesses and some other items. I encountered some issues with some of the engine senders not using the new standard resistance ranges, the old VDO instruments used different resistance ranges. So now most of the engine senders are new as well as the harnesses.

Running the boat some of you might have read my other thread about the acceleration issues I have been having. Anyways, from a performance standpoint, the following data is from the dash tachs (photo tach verified at 3000, didn't verify the others), Garmin GFS-10 fuel flow senders, and GPS.

20 knots at 2600 RPMS, 1.1-1.2 MPG

23 knots at 3000 RPMS, 1.0 MPG

27 knots at 3400 RPMS, 0.9 MPG

31 knots at 4200 RPMS, 0.8 MPG

Hope this helps someone in the future. I appreciate all of the help from the group. Plenty more to do.

Those numbers are NOT bad at all!

[Carl]

Many manufacturers over prop their boats when brand new.

Kudos for getting them propped right!


I think you may a bit off on your determination as motors need to be signed off on for warranty, but they can definitely get real close to overpropping as slow boats do not sell as well.

That is a thing to keep in mind when buying a new or used boat.


Also when buying make sure dealers are not playing games...take a ride and speed hits the mark....but a motor lags. Must be a bad tach so schedule a new sea trial with calibrated tachs...


But buyer is not aware on new sea trial a wheel has been de-pitched. Yup, you hit WOT, but didn't notice less speed. Games dealers play....so glad I am out of it.

[scenarioL113]

I agree Carl, They def want the boats as fast as they can make them when new and being trialed.

Problem is when they are being used they are not at their "weight as used" (people, ice, food, etc..)

Maybe its the owners that are overloading them to where they become overpropped.