VIC
5904 posts
to move a 14 ton vessel through an opposing current of say 3-5 knots ( these conditions would be quite common while cruising around coastal areas i would recon, in and out of ports etc ) at a speed of say 3 knots . means we have to have to boat moving at 6 plus knots ?
what do you recon would be the minimum HP to achieve say 3knots in adverse conditions in a 14 ton yacht ?
the reason for asking is , i have no idea !!!! and the next yacht i own must have plenty of HP ,as i believe for cruising that is a high priority even in a SV.
ps that's why S/Aces post with 150 hp radford was interesting to me . a motor sailor similar to that would be the grouse i recon .............
NSW
2711 posts
I have read/heard that 4hp/ton is the minimum recommended. MB has 20hp and weighs over 6 tons. She reaches hull speed at near full power but could do with say 24hp. Original S&s34 spec was only 16hp. My sisters Valiant is 12 tons and has I think 40-44hp (Perkins 4108). It is a bit underpowered. These 2 examples support the 4hp/ton theory.
NSW
7757 posts
Morningbird has it about right. The important thing though is the propeller. All propellers are very inefficient but if your going to motor sail you will need a propeller that's not the most inefficient. Largest diameter you can get away with. 3 or 4 blades.
1229 posts
Around 60 hp should give you good motoring performance
Equally important is that the power and rpm is well matched to the propellor of the proper size
Too much power is a handicap as the engine needs to work efficiently or it will give problems
A displacement hull will only travel at displacement speeds regardless of power
Having some extra horsepower in reserve is handy though for adverse conditions but not much more than 10 %
WA
405 posts
Hi SandS.
My 42' steely's displacement is 14t.
It has a 65hp Perkins and I'm really happy with it's performance. But as mentioned, prop size and type is the other major consideration. I have an autostream feathering prop and the feedback to the helm is very positive.
At 1600 rpm she cruises well at about 6 knots.
A few yachts of similar size I have been on and seen for sale, have only a 40hp motor and I don't think I would be too confident taking that type of power to weight ratio anywhere were there are strong currents, say up north.
NSW
1322 posts
Apart from the tonnage, there are issues of hull windage above the water, and shape and wetted area below. Then, like others say, you have to get the engine power into the water, and the right prop for the job is needed, including matching pitch to engine power curve and boat hull speed.
Many older boats on the market are under-powered, the reasoning being the motor is only there to propel the boat when there is no wind. If you take the approach that one day you might need it to help claw off a lee shore in a 40 knot breeze and head sea, it is another dimension altogether.
So the right power requirement is specific to the boat and your intended use. Thats how naval architects earn their bread.
VIC
5904 posts
thanks MB thanks everyone , it seems the 4 hp per ton rule has the tick from most of you ............ its great what can be learned from seabreeze !!!!
QLD
998 posts
Thought I'd chime in just out of interest for the equation. 2t, 26ft, 9.9hp. Hull speed at 3/4 throttle. Any more than that and the bow lifts up and the bum digs in.
VIC
5814 posts
Some one told me on the H28
Any more than 24 hp and just like slamin said the bum digs in if you have a 30 hp
Hull speed rules
SA
88 posts
I agree with the 4hp/tonne as a rough rule of thumb. 3hp would be enough in calm conditions but 4 gives you al little extra for dealing with wind.
Keep in mind that when running against a 5 knot current, even with extra power you may not be able to do much more than 2 knots on a 34 foot boat which will do 7-8 knots in calm water.
The theoretical maximum speed for displacement hulls is for speed in the water. A bigger engine might get you an extra half knot, but not much more.
It will make a much bigger difference motoring into the wind.
I went with 6 hp/tonne when repowering my pilothouse motorsailer. It made a big difference (2 knots) powering into the wind, but not so much (<1 knot) against the current.
