Narrow beam and flat bottom...compromises

[skaraborgcraft]

Skin on. Advantage of a model, i can see how many plywood sheets it would need. I know programmes spit out this info, but good to know i can fill in the bottom with under 5 sheets.

 

[Milehog]

My little 17' fiberglass traditionally designed sailboat has a low displacement box keel.
My first impression was that displacement down there made the boat initially more tender than I expected. Let's just say the first time I boarded her on the water I put on a show for my dock mates...
Now I appreciate the room for moveable ballast to adjust trim, ground tackle, wet boots, beach rollers, etc. etc.

 

[skaraborgcraft]

My first impression was that displacement down there made the boat initially more tender than I expected. .

When i first float tested a 13ft scale model, i had the same view. As i discovered again today, a lot of weight down there is needed.

 

[skaraborgcraft]

So, after getting my scale 1:12 cubed number, 1728; i could move on to water tank and flotation.
Model dry weight was a scale 2050lbs for a bare hull, which is close enough ball park.
I added a scale weight engine of 250lb and a ballast load of 420lb, and she was high in the water but stable.
It took the equivalent of an extra 3360lbs to get her down on the waterline, with no transom immersion. All good numbers, though a 2.8 tonnes displacement is more than required for the intended use, if the stability is there with a lighter load, all good.
Due to a tank leak, i had to work quick and no pics. I will get onto a stability test next.

 

[skaraborgcraft]

shade tree hydrostatics.

 

[skaraborgcraft]

15 degree heel took the equivalent of 151kg/ 332lbs on the rail,
25 degree took 295kg/ 645lb, and just submerged the transom chine.
There was positive righting all the way up to the edge of the deck.
My test tank (planter) is not deep enough for me to add a ballasted bulb under, but somewhere i have seen the math for working the lever arm of a 250lb weight on a 4 ft lever.

Even with the projected load, there is 500kg spare displacement to allow for heavier than expected hull, or internal only ballast/extra tankage etc.