Marinette Boat Forum

I have a 28ft Marinette Sportsman single engine (currently Chrysler 360 v8 275hp 5.9l reverse rotation). I want to re-power to a MerCruiser engine for its reliability and parts availability and to get some more HP / speed. I run scuba diving charters so at times I add a lot of weight (let's say with 6 divers, me and my dog it can be 2,000lbs of extra weight). I am trying to decide the best engine option to haul this much weight around and be able to get on plane. I don't have to go super fast - if I can reach 20mph cruising speed that would be great, 25mph ideal. Has anyone put a Mercruiser big block, like the 357 Horizon Inboard, in a 28ft Marinette? Does it even fit? Is it worth it? It seems like a switch to EFI big block is going to be rather pricey (at least $25k when all is done). That's why I am questioning if it's worth it or if I might also benefit from staying with a small block and take advantage of a MerCruiser engine reliability and parts availability, e.g., get a MerCruiser small block (like 5.7L 357 Magnum Bravo 4V) and possibly still be able to use my Velvet transmission. I don't like carburetors necessarily but on the other hand I don't like to add that many electronic components knowing how the marine environment is harsh, damp, and that electronics don't weather that well in that kind of environment so basically I'd be adding a different level of complexity. I am also not sure the return on investment is there for the EFI vs. carburetor option. Are there other options to consider? Diesel conversion seemed rather pricey as well (though who wouldn't want a diesel for a work boat)?

I would think that the speed would be close to what it is now with a 5.7 Chevy Mercruiser Horizon inboard. I would think that 20 mph would probably be on the high end with 6 people. A big block would be more power, but also more weight, and then you have other issues, like bigger shafts, props, and transmissions. That being said, my hope is to someday change out my 318's with a pair of 350's, but the difference is twins in mine verse your single. Good Luck with whatever you decide.

It's all about prop shaft rpm. If your boat has the 1.52:1 transmission WITH the 1 1/4 " driveshaft, then you're in good shape. But if yours has the 1:1 trans and that skinny 1 " prop shaft, you'll have massive propeller problems with significantly more horsepower.

A few of us have been down that road, and its a bumpy one!

Which have you?

Jeff

It is 1:1 with a 1in shaft so I know I'd have to make some changes there too if going to a big block.

It's all about prop shaft rpm. If your boat has the 1.52:1 transmission WITH the 1 1/4 " driveshaft, then you're in good shape. But if yours has the 1:1 trans and that skinny 1 " prop shaft, you'll have massive propeller problems with significantly more horsepower.

Jeff - maybe a different topic but can you expand on this issue? I have a single Chrysler 440 with a 1:1 velvet and 1 inch shaft. Prop is 14x11. This exact setup is listed for a 28' Sportsman in the document that has been posted several times on this site with all the original prop sizes.

I have had the boat 3 years, seems to run fine. Probably not the most efficient setup. I've heard of issues with prop spinning too fast, cavitation, etc. I would guess the torque of the 440 would be better put to use by the larger shaft and reduction gear...but my boat hasn't exploded or anything. Just curious to hear about your experiences in this area.

Thanks!

Others (John Hall) on this site (and Diver Dan) repowered their 32 footer twin-engined big Ms with uprated engines. John used 325 hp Chevies and had to buy 4 blade specials from Australia to get her right. Dan (the mad fiend!) repowered with big block Chevies (that intruded into the cabin) and never got the props to work correctly.

Here's the deal: If you have a moderate amount of torque/ hp you can get away with a 1:1 shaft. But if you go the Diver Dan route, with massive power and torque, you'll blow away the props and go no faster.

Hope this lousy explanation helps.

Jeff

Jeff is right on as always.

These boats were very well engineered to begin with. Get too far out of the design, the performance curve drops like a rock. Enjoy your Big M and be prepared for disappointment if you desire to make a race boat our of her.

I would scale the engines down if I could for mine and go slow and enjoy our time on the water.

To each their own as they say "Its your dime"

safe harbors
dave

Naval engineering is one of the most fascinating subjects for someone interested in modifying boats in any way. When I was just out of high school, and starting my Thompson project, my parents bought me a book, Inboard Motor Installations, by Glen L. Witt/Ken Hankinson. Although a bit dated, and a collection of brief subjects, it sparked my interest in hull design vs. power requirements vs. desired performance. I recommend this book as a starting point for anybody looking at modifying their drive train in a boat. After reading the first few chapters, anybody should have a good idea of what they desire from their vessel, and what it will take (if possible) to make it happen.

Just purchased this book off Amazon, good tip. After buying an old Marinette I've had some on going engine issues and the subject of re power is in the back of my mind. From everything researched a stock application is hard to beat, adding 10%-20% hp is doable but anything over that you begin to run into dark territory.

..."Naval engineering is one of the most fascinating subjects"

You said it! Before I discovered the Wonderful World of Marinettes, I had an old Bayliner 28 footer. It would cruise all day at about 25 mph (3,500 rpm), but that SOB would not go over 29 mph no matter what I did. Threw a whole bunch of props at it (325 hp repower/ Volvo-Penta) to no avail; 29 mph was it, period.

Across the harbor was a guy with a similar boat, but with a dead engine. He found a big block Chevy for it dirt cheap, so in she went. Well, guess what happened? That SOB never went over 29 mph either! Like mine, his hull hit a brick wall at that speed that a nuclear reactor couldn't push through.

John Smith (his real name--honest) then arrived at our slip with a 15 year newer, 28 foot, Bayliner. Had a SB Chevy (around 300 hp) and a Mercruiser Alpha One. Boat hit nearly 40 mph! What the hell, thought I, and decided to find out why it performed so much better. At winter haul out I crawled under both boats and photographed the hell out of the hull designs. Other than a different number of strakes, they "looked" the same, so...

Dammed if I know!

Jeff