Why Don't More People Use GM's 6.5 Diesel for Repowers ???

Natchamp · Post by **Natchamp** » Sat Jul 16, 2011 10:44 am

Gents,

Just curious why more people don't use these motors for diesel repowering. I know you will read a lot of negative opinions on the early 6.2 and some of the 6.5 diesels but you also see a lot of positive as well. There are comapnies that have developed solutions for the earlier shortcoming of these motors.

Good points: They are low cost to procure. They will bolt up to, without modification, the standard GM bellhousings (ie Crusader 454's). There are readily available marinization components like watered exhaust manifolds, etc. The overall engine size is very equivilant to the Crusader's.

The only issue I can think of (which I'm not sure on) is the counter rotation aspect.

For example, let's say you have twin Crusader 454's with Velvet drives. Am I correct that you would still need to install new transmissions because of the counter rotational issue? Or could you somehow use the Velvets? I read somewhere about the rotational setting "arrow direction on the Velvet" where you simply rotate the Velvet transmission corresponding to the motors rotation.

If you built the 6.5's torque to resemble the 454 you should not have a problem with the Velvet's strength.

What am I missing? I'm not saying I want to do this but it sure does seem like a good alternative for a lot of older boats out there.

Mark

prowlersfish · Post by **prowlersfish** » Sat Jul 16, 2011 6:13 pm

I know the 6.2 and the 6.5 well . I happen to like them.They are a very durable P/U and light duty truck engine . Now as a marine engine they ask to do more then they were built for . the HP of the 6.2-6.5 was 130 to 165 , ( 165 was military only) . would you want to replace your 350 hp gaser with 200 less HP ? most likely not at this level the would hold up fine. Now the turbo did put out more HP 180 to 215 hp . still a lot lower then a stock big block. you would have to run this engine very hard to get a decent cruse speed , durability issue will start to show up. there are some marine ones out putting out 250 -350 hp and more , this is pushing the engine beyond what they where ever made for , yes the made some improvements and are made by AMG now , but the are still based on the same designed . the few ones I have seen in marine use had not done all that well , look at a Cummins crank and then the 6.5 . Big difference and thats just the start .

Price out one of these "Improved"

marine 6.5 they are far from cheap.

Unless you can live with well under 200hp per engine , It would not be a good choice

As far as the Velvet drive goes you will need to replace 1 to have counter rotating props , you can turn the pump to match the engines but you would have the same rotation props , you can get a velvet drive that will counter rotate I believe its only the 1.91 or 2.10 to 1 ratio only ( I could check .)

pk · Post by pk » Sat Jul 16, 2011 6:14 pm

IÂ´d rather go for Ford / International / Navistar 7.3 with turbos. They are more reliable, you can take out more power, and in the end I think they are more economical to the fuel.

prowlersfish · Post by **prowlersfish** » Sat Jul 16, 2011 6:25 pm

No real difference in fuel burn on any 4 stroke diesel all burn almost the same per given HP .

the 7.3 doesn't have that grate of a track record in marine and marine parts are $$$ and hard to get over here ( the US )
the have had some injector and valve issues , maybe just the early ones as they where not in the marine markit for long over here

Natchamp · Post by **Natchamp** » Sat Jul 16, 2011 9:29 pm

Hi Paul,

Thanks for the info. I certainly don't know too much about diesel's. I always played with gas engines.

My follow up question is..........what about torque?

It seems like everyone always talks about HP but torque is the real key factor. Certainly I would think the marine environment (low rpm's, heavy load) would be favorable to a torque. That's one of the major benefits of diesel, is it not?

If you compare the torque between the 6.5 and the crusader, wouldn't the 6.5 win that war?

Mark

RWS · Post by **RWS** » Sat Jul 16, 2011 9:56 pm

Here are two articles that apply specifically to the 10 Meter series with diesel repower.

Take these with one or more grains of salt as the market was quite different in 2009, hence the "numbers" part of the argument may not be 100% accurate today.

http://www.boattest.com/resources/view_ ... ewsID=3361

http://www.boattest.com/Resources/view_ ... ewsID=3790

RWS

prowlersfish · Post by **prowlersfish** » Sat Jul 16, 2011 10:51 pm

I can safely say those figures from gas to diesel are not real life .Remember they are from a yanmar dealer , so chock it up to trying sell you .

Yes torque is important , HP is torque X rpm . My Cummins are 300hp at 2800 rpm , and have more torque then a Yanmar thats 300 hp @ 3800 rpm . yet both are 300 hp. What engine will do better ? They both will do about the same . by using different gearing both end up with about the same torque at the prop. HP is the bottom line . the advantage with a diesel is not in the top end but in the cruse speed as you can run it closer to top . My boat tops out at 27 knots and cruises at 22-24 . the same boat with Gas (330 hp ) will top out well over 30 knots yet the cruse will be in 17-18 knot range. the same boat with 200 hp diesels ( cummins V8 ) will top out 18 knots and cruse maybe 14-15 knots .

so you can repower with diesels and use less HP and keep your cruising speed , but to repower with half the HP is asking to make your boat a slug

RWS had 300-330 hp gasers and when to 315 Diesels a good choice 170 would not have cut it .
My boat when fron 330hp gas to 300 hp diesels loss of top eng but incress of cruse speed and range . 250 HP would have worked well also I know of many have gone that route much less and and you won't be happy .

Natchamp · Post by **Natchamp** » Sat Jul 16, 2011 10:56 pm

Those are good articles. I especially like how they are specific to the 10m Trojan. It was interesting to see the HP rating of the yanmars at only 170 and 180. Which kind of proves my ponit (unless I'm still missing something).

Even though the top speed didn't improve very much the improved fuel economy was substantial.

Mark

prowlersfish · Post by **prowlersfish** » Sat Jul 16, 2011 11:07 pm

I added to my post above .

They lost speed not gained and my point is 170 hp won't cut it . way to little power for a 10 meter you'll end up running it way to hard and a short life . Sales Hype . load up that boat get a liltle gowth and you will be lucky to get on plane

Natchamp · Post by **Natchamp** » Sat Jul 16, 2011 11:13 pm

It sounds to me like you have a horse in the race.

You're telling me both of those documented articles are BS? You don't think a company like yanmar would be more on the causious side when there products are represented? The last thing they want is a customer seeing an article like that, spending 80k and have their product not meet the customers expectations. Even if the articles were both "flavored" on the positive side they still point to dramatically better fuel economy with 180 "HP" diesels.

My whole point about the torque was for low end (as I said) which is exactly what we ask for in a marine motor.

Do you know what the rated torque is for the 454 crusaders?

prowlersfish · Post by **prowlersfish** » Sat Jul 16, 2011 11:23 pm

I Ran the numbers on the boat diesel calulater and if I was to use 170 HP diesels I would lose 7 knots giving me a top of 20 knots and my boat is easier to push then a meter boat .

I say their numbers are a strech ( light boat fresh bottom etc ) not real life and that info is from Mack Boring a dist, not yanmar it self

Heres a repower of a 10 meter with 345 HP

http://boatdiesel.com/Articles/Articles ... uction.cfm

prowlersfish · Post by **prowlersfish** » Sat Jul 16, 2011 11:50 pm

Natchamp wrote:It sounds to me like you have a horse in the race.

Do you know what the rated torque is for the 454 crusaders?

Do I have a horse in the race ? no But I would hate for someone to make a costly mistake and under power a boat .

Do I know what the rated torque is for the 454 crusaders?
I would guess in the 450 range , Most likely higher then both the yanmar 170 or the 6.5 ( under 200 hp ) remember these are higher rpm diesels 3600 pm ( 6.5) so the would be more in line to a gasser then say a 200 hp DD @ 1950 rpm .
Remember props move boats so its the torque that gets to the prop that counts and gearing can make the difference

RWS · Post by **RWS** » Sun Jul 17, 2011 7:02 am

Even though I have the 315's, the 370 was not that much more and may have been am even better choice.

According to the NUMBERS we should have had a 25-27 kt cruise and a 34 + - WOT

Our real numbers are a 20-23 kt cruise and 27.1 kts WOT

The difference is thge actual boat weioght vs. the published numbers.

Published numbers show the 10 meter express at 12,000 lbs. dry.

Even with the lighter Yanmars and lighter genny, we weighed at nearly 18,000 fully loaded with all gear and spares.

The Trojan published numbers are for a base boat. No additional weight for all that deck furniture, freezer, fridge, icemaker, windlass, microwave, range, oven, tools, spares and the spare shaft & props that we take along on a long trip.

In my experience, you could get away with a smaller diesel, as they suggest in the article, but in the long run for the difference, it makes better sense to stay around the 300 HP number as the boat was originally designed from the factory.

Don't know the numbers but probably about the same difference iun running this boat with 454's vs. smallblocks.

FWIW, my most efficient cruise speed was about 15 kts with the 454's yielding .75 - .81 nmpg in smooth sea conditions with a half tank of fuel.

These are respectable numbers for a vessel of this size and girth.

Play nice now.

RWS

RWS · Post by **RWS** » Sun Jul 17, 2011 7:20 am

another 10 meter test (mid cabin)

This is a contemporary test that comes with a video as well.

http://www.usboattest.com/test/604/1987 ... cabin.html

RWS

prowlersfish · Post by **prowlersfish** » Sun Jul 17, 2011 8:55 am

One thing you will notice , the Mack boring data is in MPH , Both myself and RWS are talking knots , that can throw you off . ( 24 mph = just under 21 knots ) so that makes their numbers a closer to what they should . ( with clean light boat ) . real world conditions will be less .

although I feel 170hp would be under power the 10 meter you will save fuel (lots) 170 yanmar is a much better choice in a marine engine . remember resale too , slower / under powered boats do not sell as well .

Not try to argue with anyone just say what I have found with gas and diesel boats , over the last 30 years , and even longer on trucks and cars