HP requirement?

[jdc]

My thoughts also
Certainly doesn't seem to relate with a "slippery yacht", perhaps good for a motorboat/ fishing boat?

You're right there! I did it for a long keel cruising yacht (I did say it was a starting point, so I chose my boat!).

But I tried a sanity check nonetheless. At the bottom end my old Sadler 29 (which is 28' LOA actually) had 20P, and my current R42 boat has 50HP. Ex Solent Boy's R44 is over powered I think and probably only 'needs' the 65HP engine, but isn't way adrift. A big Bav has 110 HP for 55' and the Oyster 62 has 185 HP so the 'scatter' is both sides of my graph.

The main thing I wanted to convey is that 'rules' such as so much per ton or so much per foot are illogical, one has first to work out whether (and why) it should be directly proportional at all: why not a square law, or an exponential, or a log, or a factorial or some such... It's simply ignorant to assume direct proportionality; one needs to do some dimensional analysis. The separate considerations of driving against a strong wind and driving the hull through the water are not scale invariant.

 

[William_H]

If you designed a boat a large weight but nice underwater profile and no upper body then a very small horsepower would move it usefully. In practice it is head wind that needs the power. (depending then on wind resistance)
I have towed 4 additional 24fter plus my 21fter at 4 knots with a 6HP o/b no where near full power. But that was in dead calm and took a while to get up speed.
So the 1hp per ton may be as good a guide as any for practical boating.
Re the "hull speed of the Feeling 720" The given 6knots is a good number. However hull speed is not a finite speed. It is useful for comparing the relative hull speeds of boats of different water line length. The "hull speed" will be achieved with quite small engine power. However the "hull speed" really describes a part of the power/speed curve(graph) where increase in power provides less increase in speed the faster you go. It is a point on a hump.
The shape of this hump or just how dramatic the hull speed limitation is, is determined by mass of the boat and mostly shape of the hull. So a catamaran with fine hulls will not notice the hump as much as a heavy barge.
The hull speed hump is often referred to the size of the bow and stern waves and the hole between that a boat tends to find it self. hence the difficulty of climbing out of the hole. Obviously to get the hull to plane largely gets over the hump. As in a speed boat.
So back to the engine HP question. If you want max speed ie to press on into this area on the speed graph for that extra knot then you need more HP. To be happy with hull speed or less then very small engine will do. Providing you have enough power to go against a wind. good luck olewill

 

[Salty John]

You need to be aware of how you want to use the engine - you can end up with an engine that does what you want it to do for your way of sailing or you can end up with one that gives the maximum usable power. Nowadays builders tend to fit engines closer to maximum usable power, presumably in response to customer requirements - and also in view of the availability of much greater power to weight ratios in modern diesels.

The naval architect, and long distance cruiser, Tom Colvin, had some interesting observations in his "Cruising as a way of Life". He reckons that a long distance offshore cruising boat (and we're talking here about boats in the 30'LWL to 40'LWL range) would only need half a horsepower per ton because it would rarely need to motor when there was wind, only through calms. For coastal cruising he thinks 1 HP/ton is the right figure.

He makes an interesting comparison between boats of 30', 35', and 40' LWL of various displacements, comparing the speed in knots achievable with a 10, 15 and 20 HP engine to the maximum usable horsepower for these same boats. A 30' LWL boat of 10 tons, for instance, has a max. speed of 7.3 knots and a maximum usable HP of 35. A 20HP motor in such a boat would give 6.6 knots, a 10HP motor would give 5.6 knots. So, depending on your cruising style you could argue for any of these engine sizes - Colvin himself would fit the 10HP.
At the other end of this scale, a 40' LWL, 20 ton displacement yacht has a hull speed of 8.45 knots and maximum usable HP of 81. A 20HP motor would move this boat at 6.25 knots, a 10HP motor at 5.35 knots. So, again, depending on your cruising style you could choose between a 10HP and an 80HP engine for this boat!

I've been long distance, live aboard, cruising in a 32' heavy displacement boat with a Yanmar 27HP and in a 41' heavy displacement boat with a 39 HP Perkins. In neither boat did I long for more power. Hence, I think 1HP/foot is fine - and that's the average power/foot of the boats in Jimmy Cornell's World Cruising Survey of 1990. I bet it's a lot higher now.

 

[jwilson]

Had a 6 ton long-keel sailing yacht with 7 hp Volvo MD1 diesel and three-bladed prop, fine for calms and in and out of harbours. Would still just make way to windward into strong winds and corresponding seas, but very slowly indeed - much faster under sail.

Now have 27 hp on 35 ft AWB of 6 tons. Don't see a need for more power - if you try to motor hard directly in to strong wind/seas you feel the boat is going to disintegrate with the slamming - though it hasn't yet. Prefer to motor-sail with deep-reefed main only, much less slamming and just as fast VMG.

Some motor-sailer builders like LM, and Moody with boats like the Halberdier, recognised that motorsailers WILL get driven hard to windward under power only, and rigged and structured the boats to take it - at a price.