Would this boat work with a 5hp engine

[andrewallan]

Firstly, I'd like to make it clear that I have little concept of what aspect of the hull design differentiates a planing hull from a displacement hull........although I obviously understand the different outcomes.

However, I've just acquired an old Hartley style 18' half cabin boat, without motor, and need some help with the hp issue.

Apparently it previously had a 115hp motor on it. This suggests to me that it is actually a planing hull, rather than a trailer sailer/displacement hull....... or else that the previous owner was an idiot and overpowered it more than a little...........

That said, I don't have the finances to replace the 115hp motor.

I do, however, have a 5hp motor, and am wondering whether an 18' planing hull might work as a displacement hull at low speed, as my internet research to date suggests that a <8hp motor is more than adequate on a Hartley 18' displacement hull trailer sailer. I'm hoping I can "put-put" around in mine at 5-10 knots with a 5hp motor..................

My current (other) boat is a John Welsford designed (NZ) Setnet dinghy at ~3.8m, and ~1.4m beam, and whilst it does actually plane with 1-2 adults in it on a calmish sort of day with the 5hp........it ain't that grand with any significant chop.

Any thoughts would be appreciated.

Andrew Allan

[Grant Nelson]

Pictures, drawings, or links to these or the design or manufacture would help an lot, as, I at least, do not know what boat you are talking about. A search on the intent gives power and sail boats... several different models...

In anycase, 5 hp will not get you moving fast, but you will move. The bigger question is how much hull is exposed to the wind, and if you will be able to move and steer safely in any kind of stormy weather... I suspect not, but let us see a picture... best of the boat out of the water from aft angle, but the more pictures, etc. the better...

[gordanm]

Since you mentioned Hartley trailer sailer, I assume it's one of "Hartley Boat Plans".
There are actually three designs in 18 feet length:
Hartley 18 trailer sailer - displacement hull
Fisherman 18 - displacement hull
Flareline 18 - planing hull

If previous owner had 115 HP engine, it must be Flareline 18.

If powered by 5 HP engine, I expect no more than 4 knots at WOT (this is not result of calculation but rather a hunch from boat weight - 1100 lbs empty, with regard to planning hull form)

[marshmat]

If it is the Flareline: A boat like that would move with 5 hp, but would not be easily controllable in any kind of wind or waves with so little power. Gordanm's guess of 4 knots in calm water sounds about right. This is purely a planing hull, without much consideration for efficiency or handling at low speeds. It is meant to cruise at higher speeds, slowing below 10-15 knots or so only when anchoring, fishing or docking.