New Member w/ Re-power Question

Hello everyone; yes I'm another newbie, although, I've read many of your posts over the years, when I was shopping for my current love, a 1981 34', down galley, single screw.
So, I'll now get right to the point.
I would love hear from any of you regarding the idea to replace my Cat 3208NA with a Cat 3208TA 375HP. I have the opportunity to purchase a 1992 with ridiculously low hours (100), as it's been stored on it's boat in dry dock since the late 90's. Obviously, my biggest concern is whether that is too much horse power for the prop shaft, strut, bearings, etc.? Let me also say, that as far as I know my boat was built with exactly half the drive train as one with a Cat 3208NA twin set-up.
Any insight you all may have will be much appreciated.
Thank you all in advance!

Well, my main concern would be having too much hp for that boat. But if you say that the hull was built for two Cat 3208NAs then I guess not as that is 440 hp.


You will need to change the prop and since the new prop will absorb more hp and torque you will probably need a new prop shaft and strut (if a larger cutlass bearing won't fit the existing one).


Think about this: The Cat 375 hp 3208TA should probably not be cruised at much more hp than your current NA engine puts out at wot; at best a knot or so more than where your current engine will push you. So if that is what you want, ok.


David

Why, way more work than you think. Unless the current engine is in bad shape, why.

Why would you want to spend so much for so little. Enjoy what it is.

Need to know what kind of hull. Some hulls can not put the added hp to good use, the 375 might just make it plow harder and faster with little benefit. If it can plane out, then ok.

Thanks David & Scary for your replys;your right that the new engine should not be cruised at more than the old's WOT, however, at that speed (2000 rpm or so) the boat is now up on plane; something that it's not currently able to do, and the fuel consumption per mile drops considerably, not to mention elapsed time.
Scary, the existing engine is original, and not well cared for. I'm concerned about long range trips down the Baja coast, and the ability to out run weather. The boat currently cruises at 8.9 knots at 2000 rpm, but is not capable of but another knot at WOT.
This I've concluded is because it is already well exceeding hull speed, but not planing.
Initially, I didn't care about achieving a plane, until I read a boat test showing markedly better mpg at greater hull plane speeds.
Do you think I'm on the right track?

What hull do you have?

Generally speaking, a power package is designed for a specific purpose and boat. A builder would not usually install shafts, struts, etc to cover the full spectrum of engine options. You need to know your shaft size and rating befor pulling the trigger on an engine with a lot more HP. On the other hand, derating the 3208 is fairly straight forward. The 375 hp is a great engine, if you can utilize the power. If the shaft is not big/strong enough you may be able to use a different gear ratio to lessen the shaft torque. Another thing to be aware of is exhaust sizing, the 375 is going to need much larger. Personally, I wouldnt pull out a 3208 and go back with another. A Cat 3126 or even better, a Cummins C series would be a much better choice. Even a Cummins B at 330 hp would be better.

Haywayn said:

Scary, the existing engine is original, and not well cared for. I'm concerned about long range trips down the Baja coast, and the ability to out run weather. The boat currently cruises at 8.9 knots at 2000 rpm, but is not capable of but another knot at WOT.

Do you think I'm on the right track?

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Presumably another option is to simply tune or rebuild the engine you have?

Depends on what "not well cared for" might mean, but in the grand scheme of things... comparing rebuild costs to replace costs might be a useful exercise.

Especially if it means you might only need a valve job and replacing some (all?) of the bolt-ons...

Although fresh paint is nice, too.

-Chris

At best you can cruise at 12 kts. If you can get there while producing 220 hp it will be at about 1 NM/gal. That is what my Mainship 34T did with its single 370 hp Yanmar.


But I am a little puzzled. You say that the existing engine will only push the boat to 9 kts and it is not on plane then. You are expecting then that the new 375 hp engine's additional torque will get you up on plane, and then you can back down to near the hp that the NA puts out at wot and will be going faster than 9 kts. Am I on the right track?

If so it is possible that more hp and torque will get you up on plane where the NA won't, but I think that you are overestimating the fuel mileage benefits of being there.

I have been a subscriber to boattests.com for many years. Everything I have seen indicates that their tests are on the up and up. Rarely do I see a speed vs mpg curve that increases significantly once the boat gets up on plane. I have seen a few where the mpg drops significantly while "getting over the hump". That is because the boat is very inefficient while pushing that bow wave until it gets up on top.

Planning hulls are the type that is most likely to see a big change in mpg when they get over the hump, principally because they finally get up on top and can cleanly plane.

But semi-displacement hulls like yours never cleanly plane. They sort of mush forward.

Go to https://dl.dropboxusercontent.com/u/256890/www.mainship.com/fueleconomy/yanmar2.html
to see the data for my 34T. It is optimistic by about 10% since their test boat was near empty. It settles at 1.2 mpg between 10 and 18 mph and drops to 1.0 mpg at wot.


David

Yes, the 34's can plane given enough HP. There are a couple guys on this forum with the twin 3208's in the 34' and they say they run at speeds around 15 - 20 knots. I don't know if they're still active, but you may want to PM them directly. I'm thinking Capn Craig was one, but check with Flywrite. He knows everything about the 34's (although his top speed is somewhat less!).

Personal opinion: You're going to put a lot of money into your boat for little return. If you really want that kind of speed, probably best to buy a 34' with the twin Cats.

You can cruise a 375 3208 at 250hp long term. Would not want to cruise much over that. The big question is what this boat does with the 210 on the pins with light boat, does it break free and scoot? That would help figure what it does loaded with 250hp at cruise condition.

David: I'm attaching the performance specs from that test. Admittedly the test boat was running twin 200Hp Perkins, but I still think it is comparable. You see how the running angle changes dramatically after 1750 rpm indicating that it is up on plane, and the mpg improves dramatically. I hope you can read this poor copy.

ddalme; the Californian 34 has a semi-displacement hull, if I understand your question.

kulas44; the main reason I was sticking to the 3208 series is because of engine mounting, known clearances, same transmission, and a healthy respect for them. My 3208NA has impressed my immensely, and I read similar reports about the 375 version, as it is still not too taxing on the design. My engine however, returns poor oil analysis reports, and as the P.O. allowed the hobbs meter to quit, I really don't know much about it.

Greetings,
Welcome aboard. Allow me to first apologize for being so blunt but I'll also get right to the point. It will most probably cost you far, far more to retrofit your current vessel than to sell it and buy what you really should have bought in the first place.

As has been mentioned, boat manufacturers size engines and associated gear to specific models and while you may get a good deal on a larger engine the "nickel and dime" bits and pieces will soon eat any savings. "Restoring" an engine that has been sitting idle for some 15 years isn't going to be inexpensive either.

Your mention of better mpg ($$ savings) doesn't jive with the $$ it could cost to change over.

So, either rebuild what you already have and keep a better eye on the weather or start looking again. Sorry...Just my opinion.

Thanks Edelweiss, but I'm thinking half the moving parts, and similar performance.

I see what you are saying: there is quite a hump at 9 kts and 1750 rpm where the mpg drops significantly and then increases 250 rpm later at 15 kts. Much more of a hump effect than on my 34T.


So maybe you can cruise at 15 kts while pulling no more than 250 hp as Ski recommends.


David

djmarchand said:

I see what you are saying: there is quite a hump at 9 kts and 1750 rpm where the mpg drops significantly and then increases 250 rpm later at 15 kts. Much more of a hump effect than on my 34T.


So maybe you can cruise at 15 kts while pulling no more than 250 hp as Ski recommends.


David

Click to expand...

Thanks David; I think it's very possible, however it brings me back to my original concern, and that is, can my existing hardware take it?

djmarchand said:

I see what you are saying: there is quite a hump at 9 kts and 1750 rpm where the mpg drops significantly and then increases 250 rpm later at 15 kts. Much more of a hump effect than on my 34T.


So maybe you can cruise at 15 kts while pulling no more than 250 hp as Ski recommends.


David

Click to expand...

I have seen performance tests like this. It's not so much what it does beyond 1750 rpm, but that 1750 rpm is the worst efficiency for that engine and boat combination. I've seen many that have the drop right at the point before planing. Then once beyond that point the mpg flattens. While the Mainship curve is more common, your curve is not unseen. Now, typically it reflects some form of pairing of engine and boat and fuel and water and other weight and propeller that just really struggles to plane the boat so finds itself in the neverland mid range for a while. Where you see this regularly is with surface drives.

One of the first places I would look is at the props. What rpm are you reaching at WOT and what is the expected maximum rpm? How does the boat plane if everyone moves forward for the start? I'm not suggesting you start shifting weight around but I'm betting if you had more weight forward and less back you'd plane much better.

Your time to plane can't be good. The fact it doesn't plane really until about 2000 rpm is bothersome. I agree with part of your premise that it's underpowered for a planing boat, but based on those test numbers, I don't think the CAT 375 hp would necessarily solve your problem, plus it might not be necessary for your desired use.

Still,,, shaft size and exhaust size will dictate if this change will be possible. Changing shaft size to larger will not be cheap, or easy. Same for exhaust.

kulas44 said:

Still,,, shaft size and exhaust size will dictate if this change will be possible. Changing shaft size to larger will not be cheap, or easy. Same for exhaust.

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So, any thoughts on what my 1 1/2" shaft can effectively handle? Here's a shot of the hardware in question. I'm not too worried about the exhaust unless I'm missing something, because my boat has two 5" exhaust ports, and I don't see why I couldn't take the single exhaust from the 3208TA and divide it into the two current mufflers, and out the two ports.

My knee jerk reaction would be that none of that will work. Maybe with a 1 to 1 gear that 1&1/2 inch shaft would be big enough. Splitting the exhaust is a really bad idea. None of your current setup will be useable. The prop and shaft are way to small to utilize 375 hp. You really need to get a knowledgeable marine engineer or at least consult with a yard that knows what theyre doing. Can you make it work ??? yeah. Would it be right??? NO. Derating is still a viable option.

I dont know what gear you have now but its my opinion that your current setup is marginal (shaft size and exhaust) for the engine you have. Gas big blocks use that size stuff. Going up in power will be VERY expensive just in installation costs. Everything will need upgrading. Dont take my word for it, get a trusted professional opinion. They will tell you the same thing.

kulas44 said:

I dont know what gear you have now but its my opinion that your current setup is marginal (shaft size and exhaust) for the engine you have. Gas big blocks use that size stuff. Going up in power will be VERY expensive just in installation costs. Everything will need upgrading. Dont take my word for it, get a trusted professional opinion. They will tell you the same thing.

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Actually, since your last post I downloaded the "Caterpillar Marine Engine Applications & Installation Guide" Copyright 2000, and though you are spot-on about my gear being way too light for 375Hp, a 1 1/2" stainless shaft is completely appropriate for my 3208NA.
It appears that I should be looking at something like 2 1/4" shaft size.

Don't think you have enough prop diameter to utilize the HP to get the boat over the hump onto plane. Repower my downeaster from 210 to 300 HP. Had to change motor mounts (heavier engine ), shaft, shaft tube, exhaust, and the prop. Mechanic put it simply, need a bigger prop to convert HP to push.

Ted

You will probably need a 2" prop shaft for the 375 hp Cat. My 34T with the Yanmar 370 had a 2" shaft.

Your current NA 220 hp Cat probably has an 1-1/2" shaft. So it will take a new shaft, prop and strut to make this change work. That is a minimum of $5,000 for all.

David

Welcome and good luck with your re-power.

A 1.5" shaft can handle a 375 depending on gear ratio. A little on the light side, but ok at 1.5:1. Cabo sportfish with 435 3208's came factory with 1.5:1 gears and 1.75" shaft.

Thanks to all who have responded thus far.
I knew going in that my prop was no good, and I suspected that the shaft, and strut were too small. All your responses, and what I've managed to research today while this thread developed has confirmed all that.
Having said that, I'm not yet convinced that this is out of the question.
If it happens, it's going to be in a DIY storage yard with minimal out sourcing. I have a "Caterpillar go-to guy" who I will discuss the exhaust issue.
David. I think you're probably pretty close when you quote $5000. for shaft & assoc. components. That's do-able.
Swapping the engine will be no big deal, and I'm prepared to be in the yard a long time,
and I'm preparing to own the boat a long time.

Ski in NC said:

A 1.5" shaft can handle a 375 depending on gear ratio. A little on the light side, but ok at 1.5:1. Cabo sportfish with 435 3208's came factory with 1.5:1 gears and 1.75" shaft.

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That's good to know; Thanks.

Is this boat light enough to preform on plane? Getting there and performing there are two different things.

Weight becomes a much more important part of the package. The difference between planing freely and laboring to just make the grade (bow wave) is night and day.