Drifter skiff

[DriesLaas]

I would like to show my new boat in progress.
I call it the Drifter, and it is loosely based on the single person flats boats used on thin water in the US, like the Solo and the Pelican Ambush. It is however a completely new design, and I plan to use this boat as a test bed to play with several ideas.
The boat is built from plywood and epoxy, and there is nothing special about the construction.

The numbers are as follows:

LOA 4100mm
Beam1100mm
Depth at bows 430mm
Weight 75kg
Power 5 HP

I plan to use this boat mainly as a one man fishing platform, for protected waters. On a really calm day I will leave the harbor, if the King or Queen Mackerel are biting.

 

[DriesLaas]

I simplified the geometry of this boat a lot, to achieve the lowest possible build time. Much time went into the method of construction, and trying to optimise the shapes for quick building.
I cut the panels on my (homemade) CNC router, and that took about 7 hours in total. The start date was 9 December 2016. I kept a fairly accurate log of the hours spent so far, and I have now tallied about 37 hours.
Some random photos:

 

[DriesLaas]

Some questions and doubts loom large in my mind:

How will this boat handle 5HP ? It is ample for the boat, but I have no idea what the hull shape will do at full power. I may have to add strakes and take other (hitherto undefined) measures to get this thing on the plane and make it handle predictably.

I simply have no experience with this type of hull shape. Is it a dory or a skiff? I lean towards skiff, as I think a dory has narrower waterline beam, but the definitions are unclear. We do not have boats like this in South Africa !

I wonder how much she will pound in choppy water, and have resigned to this possibility (probability.)

I am concerned with how wet the ride will be, and have invested in a good foul weather jacket.

I do know that my fishing tackle box fits in the storage compartment....

 

[DriesLaas]

Here is a shot with the patient Yamaha fitted. This little motor is a proper outboard, able to take fuel from an external tank, and with a gearbox for forward / reverse. Weight is 22 kg though.

 

[PAR]

You're asking questions that should have been addressed before you started cutting plywood, don't you think?

It's a skiff.

Even with the very modest V entry, she's likely to pound in some condisions. With freeboard this low, yeah, you're going to get wet too.

You'll be very hard pressed to get this boat to plane off on 5 HP, given the loaded weights typical of this type of build. Assuming a modest load (full tank, cooler full of beer, tackle box, skipper, etc.) you can expect to run in the low teens (MPH), but with the available power, this will be hard to maintain, as waves and chop will tend to slap it off plane, pretty easily. With 10 HP you could maintain a plane and speeds will be much better too.

You may want to investigate the outboard and see if it's at the top or bottom of its engine class. It might be that the 5 HP with FNR is a the bottom of the 5-8-10 HP engine family and all you need to do is change some jets, toss in some intial timing and voila a 9.9 HP outboard.

 

[DriesLaas]

Timelapse of the first stage of the build is here:
https://www.youtube.com/watch?v=dFWfezGkgJU

 

[DriesLaas]

You're asking questions that should have been addressed before you started cutting plywood, don't you think?


It's a skiff.

Even with the very modest V entry, she's likely to pound in some condisions. With freeboard this low, yeah, you're going to get wet too.

You'll be very hard pressed to get this boat to plane off on 5 HP, given the loaded weights typical of this type of build. Assuming a modest load (full tank, cooler full of beer, tackle box, skipper, etc.) you can expect to run in the low teens (MPH), but with the available power, this will be hard to maintain, as waves and chop will tend to slap it off plane, pretty easily. With 10 HP you could maintain a plane and speeds will be much better too.

You may want to investigate the outboard and see if it's at the top or bottom of its engine class. It might be that the 5 HP with FNR is a the bottom of the 5-8-10 HP engine family and all you need to do is change some jets, toss in some intial timing and voila a 9.9 HP outboard.

I find myself using questions as an excuse to procrastinate. Having no opportunity to test drive a similar hull before designing, I had to take some guesses. I will bear with the results, whatever they are.

The decision of the motor was actually the starting point, as it is what I have. I looked at similar boats with similar power, and decided I could live with that type of performance.
An interesting parameter is the weight/beam ratio and its' effect on planing performance, and I suspect it is important in a design such as this.
By the way here is the planned displacements:

Displacement and buoyancy calculations
Displacement
Part Mass
Hull 75.90
Motor 22
Fuel 5
Crew 75
Cargo 10

Total displacement 187.8951825

60% of displacement 112.7371095 litres of buoyancy required

Buoyancy Vol
Rear box 20.196 0.17x0.44x0.27
Bows 25 0.5x0.5x0.5x0.2
Side stbd 101.925 0.27x0.25x1.51
Side port 101.925 0.27x0.25x1.52

Total 249.046 litres available

The 60% buoyancy is the local legislative requirement.

I know this motor can't be juiced up with larger jets easily, so it is what it is for now.

The low teens is fine for me as far as speed is concerned, I trawl at 5 kts and seldom travel faster that 12 kts.

What do you think the addition of running strakes and/or chine strakes will do to the boat's ability to stay on the plane in choppy water?

I have tried to find information on this subject, and it seems to be a bit of a black art.

I also see that the later photos of the Pelican Ambush shows the addition of ventilation grooves from the chine, presumably to ventilate the bottom aft of the boat, and I have wondered about that. Not easy to do in a plywood boat, though.

 

[ondarvr]

I had a small wood skiff with a 6hp motor, it did get on plane, but only with one person and a light load.

As for more power...it is NEVER just the jets, if the jets are different it's because there are other differences in the motor that require different jetting.

I have no idea if the 5 and 6 hp motors of that style are similar, but sometimes they can be upgraded, but the cost can make it not worth it.

I always like a bit more freeboard on these small boats, that seems to be one of the main limiting factors in how and when the boat can be used safely.

 

[DriesLaas]

I had a small wood skiff with a 6hp motor, it did get on plane, but only with one person and a light load.

As for more power...it is NEVER just the jets, if the jets are different it's because there are other differences in the motor that require different jetting.

I have no idea if the 5 and 6 hp motors of that style are similar, but sometimes they can be upgraded, but the cost can make it not worth it.

I always like a bit more freeboard on these small boats, that seems to be one of the main limiting factors in how and when the boat can be used safely.

Thanks for the reply. I agree with everything said so far. A boat like this is an exercise in compromise, by definition.
I lifted the freeboard after a first round of consulting with various fishermen, and very glad about it now. It may still not be enough. The cockpit floor is raised somewhat above dwl, and it drains through two huge "channels" out the back. The more water is shipped, the quicker it will drain. I hope to be be safe, if not dry.
The console, which is a monstrosity at this stage, will sit on two rail in the cockpit floor, which will allow me to trim the boat properly for different weight helmsmen. Another harebrained plan.

And there are two centerboard trunks under the floor, which keeps the floor stiff but can be cut open when I want to use the boat as an outrigger canoe. Hardpoints built in now will allow me to fit aka's readily later. All these things add a bit of weight, but I can't afford different platforms to play with several ideas. So primarily a fishing boat, and hopefully a useful sailing vessel.

 

[Mr Efficiency]

PAR is right, you should have came here before starting the build. Too late to change the basics of it now.

 

[DriesLaas]

Ja I got that message in the first response. I am asking, with respect, some advice regarding strakes and chine geometry, which I would have had to add after the hull shell was built in any case. So if someone has an opinion about this, I would love to hear them and learn.

 

[tom28571]

I don't see how added strakes will affect the ability to plane at all. There is a complete lack of directional stability other than the immersed chines and it will want to slide in turns or not turn well at all. Strakes or skegs will help that as will a small fin just behind the seat to pivot the boat on.

I am not as doubtful about the ability to plane as Paul but the devil in details of weight and longitudinal balance and the true power and prop of your motor will determine that. Going to higher power such as a 10hp will certainly get it going fast but it may also be nearly unmanageable and possibly dangerous to both driver and anyone in the nearby area as shown.

 

[PAR]

My scepticism about her ability to plane, is based on her length and weight. Assuming she can manage to make over 10 knots, she'll be in full plane mode, but just barely, with no reserve for chop, wakes windage, currents, etc., so depending on trim and load, she'll have trouble maintaining plane, with the available power. This assumed about a 12' LWL and a 500 pound load, which is slightly heavier than his weight estimates. This scepticism is also based on the structures I see in the photos. It'll be interesting to see how his weight estimates, compared to the realities of the build.

There might be enough flare in the aft sections, to just let her skid off to the outside of a turn, but I'd be more concerned about ventilation. Agreed a small skeg can solve the skidding issue, but makes her deeper. Generally, this type of boat doesn't need a skeg and some skidding is just accepted, then again, most boats like this also have considerably more deadrise to help them "set" into turns and "carve" through, rather than skid. Additionally, a ventilation foil could help quite a bit too.

I think 8 to 15 HP would be ideal for this boat and I also think the engine, can be upgraded to produce more output (they all can). If the engine is al the lower end of the engine family, fairly easy, if not, more work, but still very possible. These small engines usually have a very conservative "tune" from the factory and can get 10% - 20% bumps without any real effort. With more work (read machine shop) you can add 50% to 100% to these puppies.

 

[tom28571]

Paul,

I make out the planing waterplane to be in the neighborhood of 32 sq ft. With an all up weight of 400#, which I think is very doable, bottom loading is about 12.5#/sq ft. That is pretty low, especially for a zero rocker flat bottom.

I remember a friends ugly plywood 14 footer in 1955 that would run in the mid teens with two aboard and a 6hp Wizard. It had a very deep step near mid length, completely flat aft of that and a V section forward of the step. We weighed less then and the Wizard had 6 real horses in 1955 but that boat went well.

 

[DriesLaas]

Guys thanks for the responses, it is genuinely appreciated that you take the time to consider the many questions on this forum and answer them with care.
I think going forward I will finish the boat and run her in various conditions to get an idea of the hull's idiosyncrasies, and then probably add a sketch to improve directional stability which will most likely be required.
I need to read that recent article in professional boatbuilder regarding turn stability again....