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Re: 10m mid cabin - power upgrade Crusader 8.1's

Posted: Mon Sep 09, 2019 10:44 am
by RWS
+1 on the dripless

also, while your engine room is empty - lots of LED lighting

Maybe an additional layer of sound insulation?

point of information - the dash panel for a MECHANICAL diesel remains essentially the same, however I chose to add BOOST and PYROMETER gauges to best monitor engine performance.

I am not aware of any current production lightweight diesel option, thanks to the foresight of the US EPA, and I would not consider a used diesel as no one would know how it had been "loaded" during it's previous use.

Given the limited diesel choices I am aware of today, other than a completely rebuilt diesel, I believe the 8.1's are a great choice for the 10 meter.


Re: 10m mid cabin - power upgrade Crusader 8.1's

Posted: Mon Sep 09, 2019 1:48 pm
by todd brinkerhoff
Of course, some high hp outboards may be cool $35k apiece.

Re: 10m mid cabin - power upgrade Crusader 8.1's

Posted: Thu Sep 19, 2019 5:19 am
by RWS
Found this in the files from 2004

however dated, the thread has lots of good info on the 8.1's

Just ignore the diesel stuff

Date: September 16, 2004 - 01:07 AM

Repower with 8.1 gas or diesel

Hopefully I can get some help. I want to repower a 1988.11 Meter Tojan express cruiser. The boat is about 38' with a loaded weight of 20000 lbs. Cureently the 454 Crusaders push the boat at 18kt @ 3000 rpm. The manufacturer also optioned the boat with DD 671TI 450hp. So Just about any engine could fit in the engine room. The shafts and struts were equiped with1 3/4 shafts for gas and diesel. I don't want to spend 70 or 80k on a new Cummins diesel repower so I took a look at the new 6.5L Marinediesel 300hp. Simular to the Peninsular 6.5l but with a little more torque, about 515. I would like to cruise at 23kts if possible but not sure if the Marinediesel has enough to do it. Also looking at the gas Crusader 8.1, the specs appears that it would out perform the 6.5L diesel, except in fuel economy. Does any one have any experience with repowering a boat this size with the Marinediesel or the gas 8.1. I also noticed some used diesels posted on this site. Would it be enough savings to pursue this option., and has anyone got any ideas regarding a good place to shop for a good running diesel?


Date: September 16, 2004 - 08:10 AM

I did a 1973 35 Bertram w/8.1 Mercruisers from 454s. The 8.1s are taller than the marine power 454s (I had assumed that if a 3208 could go in there these could) it was real close, had to pull insulation off the motor boxes and they clear by 3/4", still had to modify sole stringers to clear risers and ditched the plastic cover. Previous cruise numbers were ~19.5kts @ 32gph now am running ~22.5kts@30gph - avg. bottom summertime, (can cruise at lower speed am still working out the numbers, only have 65hrs on them) put 1.5" pitch in the wheels 20x18.5 w/ZF2:1 still turn about 200 over recommended WOT (27->30kt)., may add another .5" this year. Went w/Mercruisers since I could get them a bit cheaper here than the other lines plus liked the redesigned manifold/riser design and availability of the smart gauges (Donzi man at heart) from gaffrig. Alan.


Date: September 16, 2004 - 09:24 AM

It is all about service and support. I wouldn't discount the Cummin's just yet, they do have re-man'ed units at a discount that would do very nicely and with your already large shafts make for a very good match-up.

Also look at install base and resale. Hp is HP, but if after all this effort and cost you have something that is not overly accepted it will possibly hurt come time to sell.

Date: September 16, 2004 - 11:35 AM

Did you see Tony's article on the Trojan repower ... great read!

I recently repowered my Trojan F-32 with used diesels. No regrets and I saved allot but in the grand scheme of things, buying used engines was only part of the savings. Actually, there were some addional costs associated with the used engines that new engines would have negated (performing major service, paint and finding and fixing bugs that likely wouldn't exist on new eninges). The real cost savings came from doing all the work myself and becoming diesel "literate"... which was very worth while in my book.

I agree with Jack in that whatever you put in, it should be main-stream in your area from a support and recognition standpoint. Saving a few grand on an unfamiliar product may not be as econimical as you think in the long run.

As far as performance goes, this has been said here many times but can be hard to really understand. Horsepower is horsepower but , when comparing gas to diesel power, how that horsepower is applied in the real world is quite different between the 2. I throw max power specs right out the window becuase noone that I know ever uses max power for extended periods of time. Instead, I would look at where most folks cruise thier engines vs. power requirements. If your looking to cruise 23 knots, how much power does that require for your boat.? Then, take that number and see where it falls on the engines power curve and "useable" RPM. Since diesels are typically run at around 80-90% power continuosly and gas engines are typically run closer to the 50-60% range, you can see that you can get away with less diesel HP for the same given cruise speed.

As an example: lets say your boat requires 600 HP total to cruise at 23 knots. That 600 HP requirement, based on 90% power, would require diesel engines rated for 667 total hp (600/.90%=666.66). (or (334hp per engine). If you require that same HP from gas engines running at 60% then you would need 1000 total hp (600/.60%=1000) (or 500 hp per engine) .

Yes, the gas engine would have a significantly higher top speed capabilty given that there is a 40% power reserve compared to only a 10% reserve in the diesel... but of everyone I know, most guys with gas run lower speeds to save fuel... and very rarely if at all ever use the upper reqions of the engines power band. On the other hand, most diesel guys I know run 80-90% at cruise all the time and rarely pull back as long as weather, conditions and trafiic permit. Most don't even think about fuel ... not many gas guys can say that.


Date: September 16, 2004 - 12:15 PM

To add to Johns comment, my experience when running a gas boat in rougher seas is that they tend to crawl up a swell and then surf down the other side. My previous boat was a 33 ft Egg Harbor with 454 Crusaders and I would go from 10 knots to 20 knots if there was a 6-8 ft swell. It was annoying to say the least and I was constantly adjusting the throttles to keep from going to slow or too fast. I think because of the governor and the increased torque, deisels don't have the same problem. I hardly notice it on my current boat. If you run mostly in bays or semi-protected waters it will not be as much of a problem. Also, there is alway the explosion factor to worry about with gasoline.


Date: September 20, 2004 - 08:53 PM

I have a pair of 454's in a 37' steel boat (fast planing hull), approx 20000 lbs, and I am investigating diesels. I've ordered Gerr's propeller book to hopefully get a better understanding of those dynamics. Presently I get 20-22 mph cruise @ 3400 rpm with WOT @ 4400, 29-30 mph. Three blade 18.15 props, 1.51:1 tranny.

The prop and hp calcualtor here gives some indications but doesn't tell the whole story.

According to the a slip calculator the above at cruise is slipping approx 35%. At WOT about 28%.

My objective is to get a 25 kt cruise.

My first observation is that there is considerable slippage. May have to look at 2:1 tranny and larger diameter 4 blade props.

But I have been having difficulty finding out just what hp a 454 is putting out at 3400 rpm (assuming 340 hp at WOT).

Any comments?


Date: December 08, 2004 - 11:22 PM

i have 355 hp 502's and the book says 210 hp at 3200 rpm cruise, so i would expect the 454's to put out around 205 hp @ 3200 obviously a little higher @ 3400rpm, hope this gives you an idea of where to start.


Date: December 09, 2004 - 01:51 AM

Thanks for the info.

I have been digging a little deeper and it appears that there is a variance in the reported HP of 454's at cruise (32-3400). Spoke with some Merc reps down at Ft Lauderdale boat show but they couldn't shed too much light however they did have some nice looking info regarding the 8.1's.

For a rough comparison (so far), I have come up with the following numbers:

GM 6.5L diesel (Hammerhead)

Cruise @ 80% WOT 2880rpm

HP 275

Torque 475

8.1 MPI

Cruise 3200rpm

HP 305

Torque 492

454 Carburated

3400 rpm

HP 275

Torque 435

One of the earlier comments indicated that gas engines run at 60% of WOT. I would suggest that they run more efficently at 70-75%.

If the info above is at all accurate then it would indicate that the 6.5L diesel would put more power to the prop at cruise then a 454 but would fall short of the 8.1.

Over the freezing months I am hoping to find out if I can turn that excessive slippage generated by my current props/tannies into better cruise efficiency with my existing 454s. I have been working through the calculators etc but it doesn't provide a clear picture (if one exists!). Spoke with Michigan prop and they put my numbers thru their program and they came up with going to a 4 blade 18.14 with med cup. (adds an ear and drops the pitch by 1" but puts in a cup). The investment here would be $1800. I want to further investiagte the effects of a 2:1 tranny might be. I realize I am borderline in terms of power.

The comments in other posts related to availabilty and ease of service are quite important I believe. I can rebuild 454s pretty cheap. diesels are another story. The new 8.1s are 'plug and play' with good parts availability. From a cost standpoint, the 454s are cheap, new 8.1s are after that, diesel come at a higher price. Used diesels - OK if you know how or have somebody reasonable to repair them. Rebuild costs jump dramatically for most diesels from what I understand.

My application is for a fishing charter boat where in one trip I would cruise for maybe 1.5 hrs and troll for 5 hrs. I'm putting on a 60hp high thrust yamaha 4 stroke gas outboard to handle the trolling - I have trolled on one engine (454) in the past for many years but I think this is where I'll find the biggest savings. For cruise speed I may do 250hrs/yr. Not near enough hours to justify a new diesel that I can see.

My 454s burn 17gph at 3400rpm. Yup, not the greatest but the damn engines are relatively dirt cheap to rebuild and maintain. Anyway, I'm going to work thru the calc/graphs/tannies/spreadsheets/curves/abacuses to see if I can optimize the 454 power to the prop at cruise speed.


Date: December 09, 2004 - 09:43 AM

I would disagree with the statement of 250 hours a year is not enough for diesels, I think you are most certainly are in the area for diesels. Nice set of Cummins 6B's (recon?) would give you much better cruise, fuel effiency and last 3-4 times longer then a gasser.


Date: December 09, 2004 - 11:20 AM

I made the statement about most folks cruising gas engines at around 60% and that has been my experience, but almost exclusively with carburated gas engines. Everyone I know cruises just before the secondaries open up. Perhaps with a fuel injected engine, 75% becomes more real.

BTW, how are you gauging your fuel consumption? Do you have flow scans? The reason I ask is that I talked to alot of people that have the same boat as me. They told me they were consuming around 8 GPH (each engine) at cruise. I had flow scans and my boat was consuming 11-13 gph per engine at cruise. I later found out these guys were estimating based on hours and gallons... when I finally talked to another guy that had flow scans too, he confirmed his boat was using around 14 gph per engine at cruise, which made me feel better.

My boat is only a 32' weighed 15000 lbs and also ran 1.5 gears... comparing that to yours , I think your numbers are excellent at 22-24 mph cruise.


Date: December 09, 2004 - 08:27 PM

I would like to repower to diesels but the numbers have to work. You mentioned reon Cummings 6BTs - what is your estimate of a recon engine? I have run a 6BTA in a 27' tow/salvage boat and I agree that they are a nice engine. The mech I was using at the time could get Volvo cores to rebuild at about $12000 CDN for ~250hp.


Date: December 09, 2004 - 08:36 PM

I ran 350CID mercs for a few years and I always cruised at 3200rpm, the secondaries opened around 3250-3300. My present boat running 340hp (reportedly) mercs, the secondaries don't open till about 3450. The experience with these is that the best fuel/speed exists if the engines are at 3400 as opposed to lower rpm - the boat runs free-er. The same was true (for me anyway) with the 350CIDs. The 17 gph/eng was calculate when moving to a different location and the boat was run at cruise for a few hours. Loaded I don't get a cruise over 21mph and it may be less in poorer conditions.


Date: December 14, 2004 - 08:44 AM

I too am working on repowering an 11 Meter Trojan Express 1984. I've decided to go with a pair of used 671ti's. Yes they are heavy,yes the weight to horsepower ratio is not as good as new engines. I'm taking my time doing the rebuild myself and my total cost estimate for install should be less than $25k. Bought the pair for $6500 w/trans, rebuild kits and additions are running about $3500 each engine ( my labor). It's the only way that I can afford it. But the boat was set up for 671ti's or 3208's. What I like about the 671's are they are a 40 year proven engine, rebuilding is very easy, and they can be rebuilt several times. I'm not looking for a higher top speed as opposed to gas, I'm looking a fuel cost and economy. My first 3 years of owning the boat cost $10k in gas alone, diesel will cut that in half or more. Considered new gas but gas at the dock is always .30 to .40 cents more (sometimes much more) than diesel. gas-diesel cost on delivery is even greater. Looked at other diesels but most have too many other issues. I wanted engines to last the rest of my life. Good luck.