Determining Hull Type;

[Gilaroo]

Thanks Par

I used her for near shore fishing , but with new engine was hoping to get out into the bay a bit quicker, putting in anything over 20hp would be cost prohibitive, the new yanmar costs are around $9000 so God knows what a 70-100hp would cost . The goal was really to see if she was at least semi displacement which it prob is so this is good news, I might achieve an extra knot or 2. Next goal is to get a new prob which suits engine best. Many thanks for input given photos not that clear

Paul

[Joakim]

The cost difference between 20 hp and a bigger one is not that big: http://www.abcpm.co.uk/engines/yanmar/

If the boat is a prismatic hull, that is not a very good one for speeds just above hull speeds. You'll need much more power than with a hull designed for semi planning condition.

Have you realized how will your fuel economy be effected by speed? You will probably get more than 5 kn at 1 l/h and at full speed (whatever that will be) the 20 hp will consume over 5 l/h.

Joakim

[TeddyDiver]

Originally Posted by Gilaroo

new yanmar costs are around $9000 so God knows what a 70-100hp would cost .

Theres a lot of cheaper brands around too. Think you get one as good half the price of Yanmar or Volvo..
I got 71hp Westerbeke with 8300e (last winter)...

[Gilaroo]

Please see attached more profile and stern photo's which will hopefully give me info on whether she is displacement, semi dispplacement or planning hull. Appreciate feedback.

[PAR]

The new photos confirm my previous contention of her hull form. The bottom paint shows some upward sweep at both ends, an aesthetic consideration. The scum line shows a nearly level chine and a the aft view shows a straight run. Once she squats down, as she approaches S/L ratios over 1.3, this chine will be set at the perfect angle to permit the boat to plane. Of course, this will require a fair amount of power to climb the hill, so you'll be stuck to displacement speeds of just under 6 knots. Tossing more power at her, but not enough to reach full plane will be wasteful of fuel and make the trim interesting, as she points her bow in the air. In other words, if you can't afford enough engine to get her up, then just accept displacement speed and save your fuel costs.

[Gilaroo]

Thanks Par, at least I know now and maybe down the line I'll put a larger engine in to get her to plan. Many thanks for your feedback. I need to get a new prop now and will order the same size as the existing one as the new engine turns opposite to the old engine.