Modification of Hartley Flareline 18

Hi all, I had recently posted on here trying to find out what type of boat I have, it turns out to be a Hartley Flareline 18. From everything I read they have a good reputation, the boat is in good condition.
I had plans to build something akin to the Atkins Ninigret at 22feet long however this boat came to me and I could not say no.
Although It was in half decent condition I decided to sand it back, refill all the nail holes and joins in order to give it a repaint, a bit of modification inside the cabin etc.
I had email Hartley to ask if this was a semi displacement hull or full planing hull but have received no reply if anyone is familiar with this boat perhaps they could shed some light?

Now onto my question. The boat has been built for an outboard although it could be built to either have inboard or outboard. I would have loved to convert it to run a small 2 cylinder blaxland motor and just chug along but I dont believe the planning hull would lend itself to it.
What I thought I might like to do is build a rear end similar to the Ninigret design that houses over the outboard motor which would effectively 'bolt up' to the existing transom. My understanding is that the plans for the boat originally allow for extension upto 1 rib - so this would effectivly be adding that length (600-800mm) onto the rear of the boat, however the outboard would remain where the transom currently is. I have seen a few boat designs where the outboard is sitting inside the aft of the boat in a motor well and not off the back of the transom.

What I wanted to know is if I did this - would it help to angle the bottom of the design up to the rear (kind of like how a displacement hull does)? In doing this, would it help it to displace water better by getting rid of the 'air bubble' that forms behind a planing hull at displacement speed?

It would be nice to enclose the outboard to reduce noise, as well as adding a little bit of extra flat deck space at the rear to stand on.
I have attached a photo of the rear of a Ninigret as well as a few shots of the boat
.

 

Your hull appears to have a planing type of hull form.
Please do just stay with an outboard motor, rather than doing a lot of conversion work in order to fit an inboard motor.
Re angling up the bottom of the proposed extension, you could do this - how big an engine are you going to fit, and what speed are you hoping to achieve?

Re your third and fourth photos, are both of these Ninigrets?
Some info about Ninigret here -
Atkin & Co. - Ninigret https://atkin.mysticseaport.org/Oar/Ninigret.html



For a start, you could perhaps build a 'bolt on' pod on each side of the engine - bolting the pods on will be much easier than trying to build them into the existing structure.
You might have to locally reinforce the existing transom a bit though, eg by adding brackets or knees. Can you post a photo (if possible) showing the transom underneath the current well for the outboard?

 

I am only wanting to put 40-60hp max outboard on it -preferably 40hp. According to the plans this is supposedly enough to tow a skier although I am not planning on doing this. The hull is for planing , as it was a little rounded out to the chine at the rear I was wondering if it was a semi planing hull.
Yes 3rd and 4h photos are of the rear of the Ninigret. Regarding the 'bolt on pod' yes that is what I am suggesting doing but rather than a seperate one on each side of the motor, it would all be one piece including the cover that goes over the motor and yes fixing it to the existing transom rather than modifying the current structure.
I will get some additional photos under the motor well of the transom.
I was thinking of angling the rear up to help it travel better at displacement speeds and to keep the rear 'overhang' out of the water at planing speeds so that it did not have the effect of pushing the bow down.

 

I once proposed a similar idea for someone who wanted a sailboat that could propelled at planing speed with a powerful outboard.

I proposed running the bottom of the boat straight aft, parallel to the waterline, then, at about one and a half time its loaded depth, after the transom, adding a straight facet, which would slope up to the loaded water line, The idea here was at non-planing speeds, the water, going under the boat would more or less make the sudden turn, and follow the slope of the facet back up to the surface. This would hopefully cut down on the non-planing turbulance. At planing speed, however, the much more rapidly moving water would more or less skip this sudden turn and break off straight aft it.

I pitced this idea as a very imperfect displacement boat as well as a very imperfect planing one, but hopefully a boat that would be better for their purposes than a pure displacement one (for sailing) or a pure planing one (for motoring).

 

Thanks for the input Sharpii - It is more or less what I am proposing as well, although I was planning on a more gentle slope upwards rather than the sudden slope as you have drawn. Do you think a less agressive slope would negatively impact the planing ride compared to what you have drawn?

 

Yes, I do.

Here, I'll explain why.

I think of the water as it makes its journey past the bottom of the boat.

First, it flows along the straight run aft.

Then it encounters the sudden turn at the facet joint. The portion flowing closest to the bottom reaches the end of the flat run, then suddenly encounters a void. Some of it gets sucked to the facet and follows it back up to the surface. But a lot of it breaks off into a cylindrical whorl. What it does is make a clockwise somersault, after which it joins the water, that made the turn, to follow the facet back up to the surface.

The problem with too fine a facet angle is that this cylindrical whorl stays small enough for most of the water, that missed the sudden turn, rejoins the water that did. This will suck the stern down, and frustrate any attempt to plane.

If the angle is steep enough, the cylindrical whorl becomes so large that the water involved in it cannot connect with the facet. If this happens, the water flowing past the whorl simply flows straight aft, so the boat can plane, especially if it manages to go faster.

But if the angle is too steep, the whorl becomes huge at very slow speeds, making the existence of the facet not much better the a vertical cut-off.

My attempt here was to split the difference. Hopefully, the hull can reach an S/L (speed/length) ratio of 1.0 to 1.20 before the whorl becomes too big. With a 9 ft Waterline this would be 3.0 to 3.6 knots. Not great, but not terrible either. This would be the top sailing speed.

Planing speed would also be slower. But hopefully not so slow as to not be worth it.

I picked the 1:1.5 slope ratio as it is close to the maximum flap angle on airplanes. It's just an educated guess.