Engine size?

[massnspace]

I am considering the purchase of a 42’ sailboat. Looking at several different ones for sale.

I am not willing to give the make as I don’t want to encourage the usual thread drift that I seem to encounter on this forum. The make is irrelevant to my question. All that matters is that is has a long keel, with a displacement of 24,000 lbs.

My question. The boat was originally fitted with a Ford 90 hp motor. Several of the ones I am looking at have been repowered with smaller engines….from about 55 to 75 hp. (One even has a 42 HP Vetus……???)

Was the 90 hp overpowered? Why the reduction? Can anyone comment what HP would be optimal for a 24,000 lb., 42’ overall hull?

Thanks,

David
Seattle

[rslifkin]

90hp is likely more than required for that boat. In some cases the choice of HP isn't because the builder felt that was exactly optimal, but because they had certain criteria for choosing what engine they installed and the choice might have been between 50 and 90hp with no suitable option in between.

[Statistical]

To push a boat with 39 foot LWL and 24,000 lbs to hull speed (8.4 knots) requires about 50 HP. Having a bit more to account for PTO for high power alternator and having additional power to fight current and rough sea state is probably not a bad idea. 42 HP is likely a bit underpowered. The original 90 HP was likely overpowered.

[leecea]

I looked at a Yacht magazine review of the Island Packet 380 which is slightly lighter and shorter. Their review said:

The 56hp Yanmar is hefty for a boat of this displacement, but with the large wetted surface area and drag from the superstructure, she needed 2,500rpm to achieve 6 knots in calm conditions, which doesn’t leave a lot of headroom for when it’s windy.

I guess my boat will barely do 6 knots at 2500rpm so not sure if that's really an issue.

[rslifkin]

Originally Posted by leecea

I looked at a Yacht magazine review of the Island Packet 380 which is slightly lighter and shorter. Their review said:



I guess my boat will barely do 6 knots at 2500rpm so not sure if that's really an issue.

Without them mentioning what max RPM is on that engine, that doesn't mean much. It could have been a bit under-propped, but RPM vs the horsepower a prop will actually absorb is exponential. So if the max 56hp is developed at 3200 RPM, 2500 RPM would only be something like 29hp delivered to the prop. So despite 2500 RPM being a significant portion of the available RPM, it's really only about 50% power.

[Iron E]

Depends:
While a 50hp would be cheaper, lighter, more fuel efficient, and ideal for long distant cruising, it may not have the power needed in challenging conditions such as inlets with strong currents, tidal ranges, and narrow passages.

[Don C L]

I knew someone years ago with a similarly sized boat and engine. He reported not much more in fuel consumption but he wanted the power available for urgent situations. That's kinda what an auxiliary is for when cruising, no?

[FPNC]

The typical sailboat can get by with 4 hp per long ton. 4.5 hp is almost always enough power. Unless you are pushing a motor sailor or have excessive windage to deal with. Without knowing the vessel, it's hard to say more.