HP required to motor at 10 kts?????

[atrometer]

At full throttle on BOTH engines you should achieve MAX rpm's (from your engine manual) on both engines. If not the props are pitched wrong.

[Amanzi]

The answer lies within the details you have supplied allowing for some small errors.
It is a fact that to double your speed you will require four times the power input all other things being equal. If one engine gives you six knots four engines will give you twelve knots. i.e. from 48hp to 192hp. It is obvious that all things are not quite equal as two engines only give you 7.5hp so either there is an extra drag factor or your figures are slightly wrong. Getting up to 10kts is going to require at least another 100hp and as the speed increases it appear there maybe further additional drag, i.e. 'squatting' (when the bows rise and the stern sits lower) which means your hulls cannot reach 10kts without beginning to plane. My opinion is that the designer/engineer who did the original calculations knew his stuff and appears to have given you an ideal set up, with a good cruising speed of 5knts and a top speed of 7.5knts which is a good top speed without being too thirsty. I have a 50ft Prout Quasar with two 30hp Yanmars and get 5knts cruising and 6-7knts top speed. My solution-- stop trying to reach for the moon- it can be very expensive.


Regards, Peter.

[mcarling]

Originally Posted by Amanzi

It is a fact that to double your speed you will require four times the power input all other things being equal.

The rule of thumb is that, below hull speed, doubling the speed of a displacement vessel requires eight times the power, however, hydrodynamics are complex and actual power required to double the speed varies quite a bit.

[yeloya]

Yr hull beam to hull length ratio seems pretty good. Helia 44 which is fatter than yours in one engine gives app. 7 kts at 2.200 RPM with 54 HP Volvo. Yanmar engines requires a bit more rev for the max torque, hence, you should be achieving this speed at 2.500 RPM. If not, then you are way too heavy..
10 kts would require min 2X100 >HP and since sail drives are not available at this range of engines, you will need to call for shaft drive and change everything on the boat. Is it worth for couple knots of more speed ?
You'd better buy a motor catamarans; they reach easily 14-15 kts of speed with relatively low consumption.


Cheers


Yeloya

[denverd0n]

It's a SAILboat. It is intended to be SAILED. What you obviously want is a POWERboat.

Seriously. Whatever your spend on these monster engines, be aware that when you get around to selling the abomination that you have created, NO ONE is going to want it. People who are looking to buy a SAILboat are going to consider your ridiculously over-sized engines to be nothing but a liability, which will have to be removed and replaced with appropriately sized engines. They are going to want a MASSIVE discount on the boat to account for the hassle you have created. And people who want to go that fast are not going to be dumb enough to buy a SAILboat. They will be buying a POWERboat.

So the only smart thing for you to do at this point is to sell the boat you have, and buy one that is actually intended for powering at higher speeds.

Good luck, whatever you end up doing.

[44'cruisingcat]

Given that the OP hasn't returned, or answered any questions, I doubt if there's much point continuing to post in this thread....

[Sardean]

Gori 2 speed variable pitch propellers might do the trick.
Run in normal for 1 engine, rough conditions or manoeuvring, run in overdrive with both engines, run 1 engine in overdrive if motorsailing.

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