Minimum Hp for 48' SD Trawler

[Nomad Willy]

Ski wrote in post # 10;
“I'd rather have the 120 sixes just because they are smoother both at low rev and higher rev. But if you can get a sweet deal on the fours, then they should work.”

I agree and I’m usually on the other side of the fence.

In this example I’d choose the six cylinder engine. Mostly the vibration of the 4cylinder that technically could be called excessive along w the 4 cylinder Cummins. Old school big fours just shake and vibe. In the 70’s when most of these boats were built very very few choose the four cylinder FL. And there were many many applications where they would have been a good choice. There are people that will say “what vibration” and if our OP is one of them fine .. but otherwise the extra cost of the extra fuel burned will worth the smoothness.

At resale many will think of the Six as proper. But probably just as many will like the lower burn rate and probably as many others will like not being underloaded 99% of the time.

The choice may have more to do w the actual engines in question. How were the fours run and maintained v/s the actual six cylinder engines chosen. Properly and totally rebuilt six’s v/s a get me by in-place refreshed engines.

Also did FF mention the International engines he frequently refers to. They are six’s and and may be a bit more modern. Some design details (not known to me) could make them a better choice. But an “odd” brand (in boats) could be less than ideal at resale. If it were me I’d put that one on the board.

 

[rolomart3]

To clear a misconception,



power = torque * RPM. If the trany can handle the increased RPM since it would definitley handle the torque, then the only requirement is to re-pitch the propos.



Here there is a choice to be made: what is more convenient, to re-pitch the propos or to change the trany? It should not be necessary to do both.


FWIW

 

[thataway]

I owned two sailboats with the 90 hp 4 cyl. Ford Lymans. I put a total of 8,000 hours on those engines,(mostly at 1600 RPM=6knots)--and no problems. Certainly sailboat hulls and rudders are entirely different. But I covered a lot of miles, and docked a number of times. I also had a Symbol Sundeck MY, with NA Cat 3208's (210 hp each). Those would get the Symbol up to a roaring 12 knots--and I ran at less than 8 knots 95% of the time--at lower RPM. I would have traded for 90's and less fuel burn.

I would vote to go with the 4 cyl. Ford 90's. The changing of the engine mounts, stringers Isi a minor issue. You can fit them right up to the current transmissions, and not change the transmissions. All of the kit will fit directly, with very minor modifications. Most bang for the buck. Run those 4 cyl. engines with the current props, and see what the boat does. Then you can re-prop, and will have some idea of what the performance/RPM/speed is. Any diver worth his salt can change out the props in the water.

Selling the boat without engines, is a bad idea. 80% of the buyers of this type of boat would be fine with the total of 180 hp, especially with twin screws--the low speed maneuvering will be with the shifters--very little throttle or rudder use required, if any.

 

[Nomad Willy]

I would add that changing the trans ratio could be a very good move.

If the six’s had/have a 2-1 then 2.57-1 ... if the six’s had/have a 2.57-1 then go to the 2.93-1. Assuming prop clearance is good. One should strive very hard to stay in the right pitch/dia ratio. Too much pitch that will lead to too much tip loss or too much dia that will lead to too much blade area and the friction from that.

A good prop man should be able to balance all these numbers.

Also the 4cyl FL’s vibration could be tamed w the right mounts and mount placements. I have Poly-Flex made in Austrailia. Some kind of plastic I think. Some apparently stiff mounts are better than the super soft mounts like Yanmars and Cushyfloat mounts.
With soft mounts a thrust bearing may be required. A thrust bearing is a good idea anyway.

 

[FF]

The simple way to contemplate a change is to figure the current shaft RPM at cruise.

Knowing the current GPH at cruse will allow a great estimate of HP required at that shaft RPM .

If your intended engine can fit a tranny gearing that will spin the shaft at current cruse RPM and meets the HP required you are home free.

The current stock reduction gears may be fine for the smaller engine as so many boats are way overpowered.

3 GPH is probably 45HP at the shaft , easy to do with an 80 HP engine , 120 or 135 is hardly needed.