designing a fast rowboat

I have a version of Drake I am playing with and the waterline dims are pretty similar to your boat. How'd you get so much volume of displacement? So far I've only gotten about 175 lbs in there!....I'm making the bottom deeper and trying to fill out the hull shape below DWL. Seems like the flat bottom helps here and helps bump up the Cp, too. Comments?

 

Well I don't have any rocker aft, which is going to increase volume and prismatic compared to something like Drake. Of course, I'm aiming mostly for flat water use, or near enough to it, much like the Savo's. Similar use case: mostly rivers and lakes.

The flat bottom is 312 mm wide (1' 1/4 "). TBH I'm currently thinking that if I do another boat in future I may avoid a flat bottom. It has some practical value but it can't be ideal for hydrodynamics.

 

Mine has little rocker aft, too. Here is where I am at. If you guys remember Drake I am after that feel, but with 4-strakes and ultralight construction in mind. This is for going fast in the ocean. At this point I have:

LOA 18 1/2'
LWL 17'
Beam 4'
Draft 5 1/4"
WLbeam 28"
Cp 0.55
WSA 28 SF
Displacement 368#

I think the planks will chop down the volume of displacement a bit. I also plan to shorten this model so my LWL is more like 16 1/2'. For fixed seat rowing, I think keeping WSA down is a benefit.

Feedback? NED: your WSA seems awfully high...conversion error?

 

Your WSA figure is wrong.

Think about it. DWL = 17 feet. BWL = 2.33 feet. Waterline area will be around 5/8 of that, give or take a little bit, so waterline area = 24.75 square feet. Actual wetted surface of the hull will be more than that, by around 10% for boats like these, so figure your WSA is actually more like 27+ square feet. In other words, about the same as mine.

Re length, bear in mind that the winning fixed seat singles in the Blackburn for years have had waterlines of 17 feet and up.

Anyway it looks like a nice boat.

 

Correection, thanks

That is called be up way too late at night! Got 28 SF, rounding up. There will be a little bit of external keel, as little as possible to get the lateral resistance I like when in open water.

I may need to skinny-up my 17' if I stick with it.

 

How about a small retractable skeg or something? That way you could have it retracted for less drag when it wasn't needed, but still have extra lateral resistance when you wanted it.

 

Skeg

Funny, I've been talking with Ben Fuller about that for years....I can see it for sure.

I'm still grappling with my hull form. I am aesthetically wed to the Drake hull shape, so I still have overhangs...I know I could get a light boat without them, but it looks so very nice! Gotta look gooraditional rowboats like the GB.d out there.

The other thing is how narrow to go on the water line...I have a bit more draft than you so don't want the thing to be too tender for people who want her as a fitness boat. LWL is a struggle too. Ben always said traditional hull forms with lwl longer than 16' in the Oarmaster trials tended to quickly become more burdensome to row, a factoid I always remembered. But I wouldn't call my boat traditional...it'll be 4mm Okoume ply and quite a bit longer than the GB's and similar craft they were testing back then. That is why I pushed my lwl up to 17'. I could probably bring the WLbeam to26"....

 

I did think about the skeg thing myself. I had an idea that a good way of doing it might be as a small daggerboard, with a short section that extended above the top of the case. The idea was the if you wanted it you'd put it in skeg down, so it was a skeg, but if you didn't want it you'd put it in upside down, so the short bit came flush with the bottom. That way it should be possible to get minimum drag when you don't want the skeg underneath, since the short bit would be shaped to fill the slot perfectly.

I know what you mean about the overhangs. I totally love the shape of the Gokstad faering for instance. Can't beat a really good sheer and a bit of overhang for looks. Weight and windage are a concern though. I did work out, with Michlet, that at speeds of 5.5 knots and up it seemed like every extra pound would cost roughly one second every mile and a half. That's a tiny amount of total resistance, but them seconds could add up if things are tight. Depends how fanatical you want to get I suppose. In practice I might be more worried about windage. That's one thing the reverse stem guideboats have going for them, even though I'm not really keen on that look.

And yeah, it depends what Ben Fuller means by "traditional". If he means something like a classic carvel Whitehall, or even a peapod, I can definitely see that being too much for one person once the waterline got over 16 feet. Might pay to get some info from him about which boats he meant, and at what speeds over what distances.

 

Ok, since I threatened to post more pictures when the thing went right side up, here they are. I've just taken it off the strongback and am rather happy. I can see the consumption of beer ensuing.

Don't know how light it is yet, but I can easily carry it on edge with one hand taking all the weight, and the other hand just being used to steady it laterally.

ETA: Got it weighed. As shown in the pics, with two temporary frames and some crossbraces still in place, it is 27.4 kg, which is a smidgeon over 60 lbs. The stuff that has to go into it (quarter knees, breasthook, one thwart in race trim, footbrace, rowlocks, paint) will probably weigh about the same as the stuff that has to come out, so that means in race trim it should be around the current weight.

 

Here's an idea for you. How about compounding the garboards slightly? Not to the extent of "torturing" them, but just enough to get some extra stiffness and displacement, and prevent panting of the panel across its width. If you are using 4mm okoume it will be a bit flexible anyway, so you could use that flexibility to your advantage during planking.

This wouldn't require much curvature, and the basic shape, with some deadrise amidships and rolling up to a stem each end, is a shape which lends itself well to slightly compounded planks. So you could make the garboards quite wide, with perhaps a light internal stringer halfway across*, and get an underwater shape that pretty closely approximates the lines shown in your attachment.


*You'd probably need this to prevent oilcanning of the panel, if it is actually compounded as opposed to just developed.

Also, I've been thinking about construction details on mine. With a bit of tweaking I think 55 lbs is feasible. The for'd half is stiffer than the aft half. I wanted a transom on this boat for a few non-speed-related reasons, but it's frankly not ideal for maximum stiffness with minimum weight. It'll work, but it's not ideal. If it was changed to being a double ender, or at least with a smaller transom that allowed more longitudinal curvature towards the stern, that would be a bonus in structural terms.

 

I may need to add some stringers, that is possible.

So here is a question: how close to bring the volume of displacement. I am at 375lbs of displacement and I don't expect me + gear + boat will be over 300. Draft is at 5 1/4" so I may take some draft out of bottom.

 

Offhand I'm not sure how much you'd lose once you take it to the plank lines. You'd just have to try it and see. Can't see it being more than 10% though.

 

I think what I need to do, is some modeling enough to get a good idea of weight and put some planks on to see if 10% is about right. I think that would be a max...will keep you posted. I am going to take a break for a bit.

Clint

 

I need a gfx app that does smooth lines. Delftship only does jaggy, which is a bit of a nuisance to work with.

And yes, I think 10% would be the upper limit. Probably closer to half that. Happy modelling.

 

gfx? What is that?

I'm in Rhino. It's wonderful but as always, GIGI principle applies.

(Garbage in, Garbage out)