Engine room 4 - What sized engine?

 What horsepower diesel engine should go into a wylo 35? (Displacement sailboat)

A general rule of thumb of "3 horsepower per ton displacement plus 3hp" (I read it long ago, in an Alan Lucas book or article somewhere?) seems to be a reasonable formula. 

(Displacement x 3) + 3 = engine horsepower

Also Eric Hiscock (p185 "Cruising Under Sail") confirms this with "3hp per ton" which allows hull-speed to be reached if needed and gives the ability to go against a headwind and sea.

So for a 7 ton wylo35.  (7x3)+3 = 24 HP. That's just the empty displacement though..

Add 2 tons of water, fuel, supplies etc makes 9 tons displacement (9x3)+3 = 30 Horsepower.

Wylo2's designer Nick Skeates states 25 horsepower maximum for the wylo32 footer (6.2 tons displacement), so 30HP for the 6.9 ton 35 footer seems reasonable to me. Going bigger than this would see a significant increase in engine dimensions/weight and costs.

Alan Lucas discusses correct motor sizing in his book "Fitting out mechanically". The engine should "not be too small or too large" for the boat. It's best to run main engines with a fair bit of loading on them. ie Don't just run it to recharge a freezer or batteries (battery charging should only be a by-product of moving the boat along, other means like solar, wind or hydro used to recharge battery's so the engine can stay off)

Most boats have a comfortable "motor-cruising speed" where not too much fuel is consummed but the boat-speed is 70-80% of ultimate hull speed. A wylo35 hull-speed is around 7 knots, so "cruise speed" should be about 5-6 knots.

It's a good idea to be able to go at near-hullspeed for short bursts. This may be what it takes to get through a fast-flowing entrance into a calm safe harbor or coral atoll's lagoon. Engines have limited rated hours for which they can do this feat. For example the Nanni N3.30 is rated to run at full throttle for 1 hour out of every 8 hours run time.

"Over-powering" results in much higher installation/runing costs/repairs and an under-utilized too large engine suffers from this misuse. Large engine boxes enroach on accomodation. They require more fuel, there is more weight aboard and less storage capacity for other things.

"Under-powering" seems to be more common.  Motor cruising speed is too slow, especially if theres a headwind/ head current.  So full throttle is perhaps used too often, resulting in more rapid engine wear and repair bills. Another problem is if your anchor drags in a gale, underpowered boats just can't make any progress against gale force winds, or may even loose steerage way altogether and blow away out of control, not good in some crowded places or near a rocky lee shore.

Some home-built boats are "overbuilt" during construction or fittout, which can raise their displacement above the designed displacement. This must be taken into consideration if buying a second hand homebuilt boats (Not usually a problem with production boats in general, unless they were originally sold at hull only stage).