10, 14 or 21hp?

[lw395]

Really? Many yachts are double enders below the waterline. Got any evidence for your assertion, because it sounds like an old wives' tale.
My double ender is extraordinarily fast, under sail or power easily able to exceed sq root of 1.3 x WL'.

I refer you to the textbooks such as Larrson and Eliasson, Bethwaite.....

 

[lw395]

While that is true to an extent, overpowering a boat is wasted. on the boat in question, maximum hull speed in flat water can be achieved with little more than 10hp. Doubling the amount of power will not enable any better performance - even in adverse conditions.
.....

'Maximum hull speed' is a bit of a falsehood.
Hull speed is an arbitry point on the drag-speed graph where drag is increasing quickly as a function of speed. There is no speed for which adding more power won't make the boat go faster.
Just 'diminishing returns' the faster you push some hulls.
Evolution of modern hulls has pushed this point up the scale somewhat, and the best modern hull will have a much less distinct hull speed hump before transitioning to planing.

 

[JumbleDuck]

While that is true to an extent, overpowering a boat is wasted. on the boat in question, maximum hull speed in flat water can be achieved with little more than 10hp. Doubling the amount of power will not enable any better performance - even in adverse conditions.

It's not a matter of doing better than hull speed in adverse conditions; it's a matter of achieving hull speed in adverse conditions.

Extra power does not help going against tide, only against wind and waves. Even then there is a limit as it is the propeller than moves the boat, not the engine. so if the propeller cannot demand the hp, the bigger engine is wasted.

Of course, and part of this exercise is having conversations with prop makers as well as engine makers.

 

[Tranona]

It's not a matter of doing better than hull speed in adverse conditions; it's a matter of achieving hull speed in adverse conditions.



Of course, and part of this exercise is having conversations with prop makers as well as engine makers.

You will discover quickly that 3 blades are better than 2 and greater blade area is also better. The Featherstream you are considering is arguably the best propeller for both efficient propulsion and lowest drag under sail - those are its biggest selling points.

However there is a limit to how fast you can make a small boat go in adverse conditions and just adding more engine power is not the answer. You need enough power to drive the best propeller for the boat.

 

[Tranona]

'Maximum hull speed' is a bit of a falsehood.
Hull speed is an arbitry point on the drag-speed graph where drag is increasing quickly as a function of speed. There is no speed for which adding more power won't make the boat go faster.
Just 'diminishing returns' the faster you push some hulls.
Evolution of modern hulls has pushed this point up the scale somewhat, and the best modern hull will have a much less distinct hull speed hump before transitioning to planing.

I am well aware of that - and different propeller calculators use different factors.

However on a 21' LWL "traditional" which the OP has, the point is likely to be somewhere between 6 and 6.5 knots and this is easily achievable with the 14hp engine proposed, given the constraints of a 13" propeller. He already achieves over 5 knots with an 9 hp (actually only 8!) and a 50% increase in power is more than enough to add that extra knot and a bit.

The discussion now is about maintaining speed in adverse conditions and that is a function of the propeller more than just adding extra power that you cannot transmit into forward motion because you do not have space to fit an appropriate propeller to use that extra hp.

 

[JumbleDuck]

You will discover quickly that 3 blades are better than 2 and greater blade area is also better.

I already have three blades!

However there is a limit to how fast you can make a small boat go in adverse conditions and just adding more engine power is not the answer. You need enough power to drive the best propeller for the boat.

Of course. I am prepared to put up with a degree of inefficiency at low power in order to produce more thrust when needed.

 

[Tranona]

I already have three blades!



Of course. I am prepared to put up with a degree of inefficiency at low power in order to produce more thrust when needed.

i know you have 3 blades - would be even worse with 2 blades. But 2 blades with 14hp would be better than now, and 3 blades with 14hp would be even better. It is all about getting the right propeller and Darglow will pitch it right for your boat/engine/reduction ratio.

Don't worry about "inefficiency" at low revs. The important thing is to fit the prop that allows the engine to develop its full power at WOT. If you do that everything else falls into place. That is you will cruise at around 2400 rpm and still have bite at low speeds to stop in reverse. If necessary you can have a steeper pitch in reverse to help.

 

[Poignard]

I have a 28-foot, 8-foot beam, long-keeled yacht weighing about 4-1/4 tons. She has a single cylinder BUKH 10hp diesel.

Last year this engine needed a new liner, piston, rings, head overhaul and, as an alternative, I investigated fitting a new BETA engine. BETA advised fitting a 20 hp engine in place of the existing 10hp.

In the event, I repaired the 10hp BUKH and a few days ago I needed to get to a scheduled bridge opening and, being late, I ran the engine much harder than I usually do. The boat speed achieved was 6 knots through the water. This was in a river.

The conclusion I draw from this is that a 20hp engine would be a waste of money, weight and space in my boat.