Engine for 30 foot old and heavy ex fishing boat

Hi,
i am a bit new with boating, having purchased my 30 foot wooden 1954 wooden boat, thats is re-built into a pleasure boat out of old fishing boat. The problem i have, is too big engine ( i know there is not such a thing as too much power, but space and noise considered, its just too much). Takes too much space, is too loud etc. So i have a boat 30 foot long ( around 26 at waterline), 11 wide ( at waterline) and it has 4,2 liter diesel engine produce by Ford lehman, 73 hp 1970 smth model. Its very heavy, and takes loads of space. I like the boat, so i decided to do some investment and thinking of changing engine into smaller one, maybe a turbo diesel, that would take less space and be less noisy. My issue is, how much can i reduce the engine size/power? I dont neet to go fast, but would like to have cruising speed of min 5 knots, with a bit power to spare.

The boat official weight ( on doccuments) is 6000 kg, so its not a light boat...

Could anyone advice me were to start and how to work out if for example 40 hp turbo diesel would be enough, also i would guess i would have to change proppeler etc as well, anything else?


Thanks in advance!

 

I can't see much sense in installing a turbo diesel in a slow heavy boat, there could also be problems matching up a suitable gearbox ratio and propellor.

 

Welcome to the forum.

Diesels are noisy, it's just the nature of the beast. Newer ones are noticeably softer spoken, but they'll still shout down a gas engine in volume. Also by their nature diesels are big and heavy. This is just the reality of a diesel, compared to a gas engine.

A turbo diesel is a smaller engine trying to be a bigger engine, with the addition of forced aspiration and higher RPM potential. They're also noisy, heavy and sizable, plus they're hot, really hot because of the turbo.

Simply put, repowering a boat is a fairly complex thing for the novice boat owner. There are several calculations that need to be performed, plus lots of little details, that need to be worked, before a reasonable selection can be made.

Lets start with what type of boat is it. What's it's current top speed, how many RPM's does it spin, the gear ratio, prop diameter and pitch, etc. With these basics you can refine your search, though 70 HP on a fat, heavy 30' boat isn't unreasonable. You might find a new engine, with 43% reduction in capacity will not do much more, than beat the water to a froth at the prop.

As far as completely changing out the engine/trans probably the shaft and wheel, well this is the long way around the bush for sure. There's lots of little details that will frustrate a professional making this type of change, let alone a novice boater. If you would feel comfortable changing out the engine, you have in the family Volvo and replacing it with one from a Fiat of similar size, including the transmission, likely custom drive shaft and other related gear (fabricating engine mounts, custom exhaust, dramatically altered cooling, etc.) then you might have the skills necessary to take on a project like this.

 

Hi,

thanks for replies. I will do some check for info. The gear is 1.96:1 PRM310 , engine Ford 2712E, engine goes up to around 2000 rpm, but on cruising i go around 1300 to 1500 as its the least noisi. Max speed around 8 knots.
I do understand that diesel is allways louder then petrol, but i am thinking that maybe its possible to get a smaller engine ( not 4,2 liter monster) to do the job.

I feel quite ok doing most of things myself, plus i have a mechanic friend, who is very handy, and in worst case some hired help. At the moment its more of a question if there is a point at all to do a change, for purely space, noise and vibration reasons? there is not enough space to built more isolated box around engine unfortunately at the moment...

 

You're not going to save as much space as you think, switching down to a smaller diesel. For example a 75 HP Kubota is 43" (1092) long x 30" (762) tall x 24" (610) wide, while a 43 HP Kubota is 37" (940) long x 26" (660) tall and 22" (559) wide. The space savings isn't especially significant. These sizes are examples, simply because I have the engine CAD files in front of me, but I'll bet they're similar to the situation you'll have.

My primary concern is driving your 6 ton craft, with only half the HP of the current engine installation.

 

Perhaps a visit to these folks can help solve the noise hassle.
.
www.soundown.com/‎
FOR PEACE AND QUIET. Worldwide Leader in Noise and. Vibration Control Engineering.