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Old 03-23-2014, 09:18 AM   #1
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Gas to Diesel

Hello.
Recently purchased 1977 34 foot tri-cabin TollyCraft with two 255 mercruiser gas engines. Engines need rebuilt. I am thinking about switching to two Perkins 4-236 85 hp diesels. What would be my cruise speed? ps I forgot to mention that the boat has velvet drives 1:52 to 1
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Old 03-23-2014, 09:47 AM   #2
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You can cruise at up to hull speed but 170 hp is not enough to get you up "over the hump" to efficient planning speeds. Hull speed on your Tolly is about sqrt (32)*1.34= 7.5 kts.

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Old 03-23-2014, 09:51 AM   #3
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I'd say pretty much hull speed - 7 knots, or pushing it maybe 8? Much above 7 and you'll probably burn a lot of fuel with little additional speed to show for it.
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Old 03-23-2014, 10:00 AM   #4
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Have you checked into the cost of this conversion? I suspect that once you realize the cost, any concern over cruise speed will go out the window.

To answer your question, your cruise speed will be hull speed or a little above.
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Old 03-23-2014, 11:56 AM   #5
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You can cruise at up to hull speed but 170 hp is not enough to get you up "over the hump" to efficient planning speeds. Hull speed on your Tolly is about sqrt (32)*1.34= 7.5 kts.

David
Using prop charts and actual test runs, I 've established that our 32,000 pound 44' OA requires about 195-200 HP for steady cruise at 13 knots. The hull is solidly over the hump....such as there is one for a semi-displacement hull. That's @ 2400 engine rpm....or 1200 prop rpm. As there isn't much of a hump, per se, it eventually get's there by just setting the rpm and waiting...and waiting... My calculations indicate it would take about 300 HP to retain a decent slow "plane" capability for this boat.

So depending on the weight and hull characteristics of the Tolly 34, it might be "possible" to achieve slow plane with the Perkins if appropriate prop and gear ratio changes were made to match the power curve for the engines. But even then the engines would almost certainly be working at their limits.

I believe someone here has a Tolly....perhaps he has some power/speed polar data for the engines and props (prop charts). That would be very useful in establishing a ball park estimate of what would be required. I'm guessing a pair of 120 diesels and some gear/prop manipulation would get a slow plane cruise capability.
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Old 03-23-2014, 01:13 PM   #6
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Unless you plan on doing the vast majority of the work yourself or you think the repower will uptick the price of your extremely sought after and very valuable boat...the normal repower that you PAY all the costs for...NEVER comes back to you....even in fuel economy unless you liveaboard and cruise full time with thousands of miles under your keel every year.

Now...if you like the diesel versus gas safety margin (which can be controlled by you but does exist), or need the slightly extended range, etc...etc...then if you don't mind throwing probably 40+ thousand dollars into a repower...then go for it...especially if you do most of the work.
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Old 03-23-2014, 01:15 PM   #7
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Here is some data from a guy who repowered a classic Mainship 34 with a Cummins 210. That Mainship is similar to your Tolly's hull type but probably lighter with less beam.

RPM Boost Kts HP GPH
1000 ----- 5.9 20 1.6
1200 ----- 6.6 30 2.0
1400 ----- 7.4 40 2.7
1600 ----- 8.0 50 3.5
1800 ----- 8.5 65 4.2
2000 -6--- 9.0 85 5.9
2200 -10- 10.2 110 7.2
2400 -14- 11.8 150 9.2
2600 -19- 14.0 210 12.0

So the twin engines that you propose installing are going to make 170 hp so it looks like you will top out at about 12 kts. You need to run those engines much lower at cruise so the best you can get is about 10 kts which is right in the middle of the hump, such as they are in semi-displacement hulls.

It probably wouldn't hurt those engines to produce 55 hp each continuously at 10 kts so you could do it if you wanted to, but that would
be your maximum safe cruise.

If you tell us what you topped out with the Mercruisers, we could give you a better guess with the Perkins.

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Old 03-23-2014, 02:05 PM   #8
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Thanks for the info. The original owner said his top speed was 26mph with the mercruisers. I will have to reconsider the 85 perkins. I also have a chance to get two 240hp turbo charged perkins with the trannys . The price is 10k and they run well but would I have to change the 1 1/4 shafts and the props or could I use the existing ones?
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Old 03-23-2014, 03:48 PM   #9
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I plugged your max wot speed with the Mercs and other numbers into boatdiesel's new calculator and it came up with a wot speed of 12.5 with the 2-85 hp Perkins. Pretty close to what I posted earlier based on the Mainship 34 data.

I plugged the 2-240 hp Perkins into the calculator and it gives a wot speed of 22 kts and a cruise of 15-17 kts.

You would need to talk to a prop shop about metallurgy, transmission ratios, prop diameter, etc to determine if the 1-1/4 inch shaft is ok.

The two high horsepower Perkins sounds like the way to go. Two remanned Cummins 6BTA-250 hp will cost about $40,000.

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Old 03-23-2014, 06:21 PM   #10
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Originally Posted by rayman62 View Post
Hello.
Recently purchased 1977 34 foot tri-cabin TollyCraft with two 255 mercruiser gas engines. Engines need rebuilt. I am thinking about switching to two Perkins 4-236 85 hp diesels. What would be my cruise speed? ps I forgot to mention that the boat has velvet drives 1:52 to 1
I have a 34 Californian with a semi-planing hull, displacing 18000 lbs, twin Perkins 4.236 85HP diesels and 2.0:1 Velvet drives. My WOT (2800 RPM) is 10 kts, cruise at 2000 RPM is about 7.5 kts. At cruise speed, I burn about 2.75 gph.

My shafts are 1 1/4 inch stainless and my props are 19" dia x 17.7" pitch.
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Old 03-25-2014, 08:00 PM   #11
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I have been looking at tollys in the 28'-34' range as that is what will fit in my slip. Great boats but they like drinking the fuel. Most I have looked at have been good shape but have lots of hours on gas engines and that gives me negotiating room. Looked into diesel repower and cost go up very quickly when all said and done. So the best option I think in this size and type of boat is repowering with new gas fuel injected v-6 or v-8 engines while not as good economy as diesel, fuel savings would increase hugely and pretty much a bolt up deal. For the price of doing used (unknown condition) diesels and getting them dialed in you could do new gas and probably have enough change left over to buy gas for a couple years. Just a thought.
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Old 03-25-2014, 08:12 PM   #12
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gas to diesel

Having lived and worked on both engines - I would not recommend going to a diesel at all.
a) Weight difference - marine diesels are considerably heavier
b) Fuel tanks - need to be changed out or added.
c) You may need to change shafts - because of vibration
d) The cost is staggering - you will never recoup the changeover
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Old 03-25-2014, 08:17 PM   #13
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d) The cost is staggering - you will never recoup the changeover
Wait you're eventually supposed to get the money back that you put in the boat?
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Old 03-25-2014, 08:24 PM   #14
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Wait you eventually get the money back that you put in the boat?
rarely....

but...if you can do the changeout yourself...are a little creative and stop worrying about "experts" in the beginning...it is all possible to do for a lot less than the fear mongers portray.

Now...if you ever had to dump the boat back on the market....it may be a stretch to get surveyors and other to bite off what you have done.

Will the boat economically get you up and down the ICW in safety? Sure...but your sweat equity and creativity are paramount.
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Old 03-25-2014, 08:49 PM   #15
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Wait you're eventually supposed to get the money back that you put in the boat?
How long is the payback on an N47? My wife would like to know.
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Old 03-25-2014, 09:13 PM   #16
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How long is the payback on an N47? My wife would like to know.
Well since you asked.
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Old 03-26-2014, 11:17 AM   #17
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rarely....

but...if you can do the changeout yourself...are a little creative and stop worrying about "experts" in the beginning...it is all possible to do for a lot less than the fear mongers portray.
Raise your hands if you are a qualified marine mechanic? Marine carpenter? Have experience in re-bedding an engine? Have any mechanical experience?

Quote:
Now...if you ever had to dump the boat back on the market....it may be a stretch to get surveyors and other to bite off what you have done.
How many times have you seen on Yachtworld and other sites - New engines, new transmission, new tanks, new shafts? A couple of months later price reduction $10-100k. That's when you as the seller have that ah-ha moment

FYI - As a notification: Capt Sam - my Father who up until this month was planing to build a metal sail cat left this world on the 21st. He had just turned 75 and now he's joined his dad in the sea. I am the 3rd generation of boat building family. As soon as I am finished with the current movement of cargo, I plan to keep up the tradition of quality woodhulls - overseas. I'd like to keep his memory alive with Ghostrider III
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Old 03-26-2014, 12:02 PM   #18
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If one has the knowledge skills and tools to swap out a gasoline engine for a diesel in a boat and have everything line up and work perfectly, his time would be better used doing this for other people. You can't just take a crane and remove the old and install the diesel, there are many, many things that would have to be changed.

As for resale value, a "homemade" engine swap of this magnitude would be a big warning for anyone who knows anything about boats. If you're going to do this you should have it done by a reputable boat yard that specializes in repowers.
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Old 03-26-2014, 12:26 PM   #19
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If one has the knowledge skills and tools to swap out a gasoline engine for a diesel in a boat and have everything line up and work perfectly, his time would be better used doing this for other people. You can't just take a crane and remove the old and install the diesel, there are many, many things that would have to be changed.

As for resale value, a "homemade" engine swap of this magnitude would be a big warning for anyone who knows anything about boats. If you're going to do this you should have it done by a reputable boat yard that specializes in repowers.
Sorry, you are not correct. My ex marina neighbor who is a carpenter, did a gas to diesel swap on a SeaRay several years ago. It was a very professional looking job and everything worked perfectly. He did not change out his fuel tanks, he used what was in the boat.
I also did a repower on my "ex", diesel to diesel, but it was a big power increase so lots had to be changed. Really not all that bad to do if taken step by step after some research.
It's more bull work than anything really.
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Old 03-26-2014, 01:03 PM   #20
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Sorry, you are not correct. My ex marina neighbor who is a carpenter, did a gas to diesel swap on a SeaRay several years ago. It was a very professional looking job and everything worked perfectly. He did not change out his fuel tanks, he used what was in the boat.
I also did a repower on my "ex", diesel to diesel, but it was a big power increase so lots had to be changed. Really not all that bad to do if taken step by step after some research.
It's more bull work than anything really.
Thanks Jay....

Some of the nicest boat work I have ever seen is done by "non-marine professionals". Sure people hack up their boats and others turn them into gems. In the long run...if you don't care about resale and keep the safety issues to a minimum...what does it really matter if you are happy with the results?

I laugh every time some poster places his limitations on me or others...and because we don't want to be full time anything but what we are doing....

Just amazing....maybe everything they do seems homemade...but I know lot's of people that achieve professional levels quite quickly.

I'm not an expert in any of those areas and I've replaced engines, trannys, built back rotten stringers, etc...etc...generally better than the way the manufacturer did it in the first place...production boats should NEVER be the standard of excellence...all but a few fall way short. Most of my knowledge has come from reading and trial and error....None of my boats have sank or had anything but "cosmetic" redos where I needed a little more attention to detail.

A great hobby is amateur boatbuilding...you know...where people have never done something before..read a little up on it and forge ahead? Some do learn quicker than others...but as jay inferred...it ain't rocket science.
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