Recreational Rowing Shell

[Willallison]

I'm thinking of building a recreational rowing shell. The boat would need to be launched and recovered from the upper deck of my parents Offshore 48. I envisage foam-core construction to keep weight to a minimum (and also to give me some experience in building with the stuff...). I have compromised on the beam as stability of typical skulls is way too low (it would be necessary to climb aboard the boat from the boarding platform of the mother-ship. Also the boat would sometimes be used in rougher than ideal conditions)

Any and all suggestions about design &/or construction would be most appreciated....

Specs of my first model are as follows:

LOA 5.0m
LWL 4.8m
BWL 0.65m
Displ. 99kg (including rower)
LCB 51% LWL
AWP 2.4m^2
Wet Surf. 2.5m^2

WP coeff. 0.7
Pris. coeff 0.6
block 0.4
D/L 23.6

[Willallison]

more

[Willallison]

and another view

[Willallison]

and the lines

[Tom_McGuinness]

That's a very nice looking hull. Close to what I've got in mind for the tailboat propulsion system (www.tailboats.com)...maybe a little more rocker.

May I ask what software you used for design?

Best regards,

Tom McGuinness

[duluthboats]

Nice looking lines. With that shape, if you go with a foam core, you’ll need to make a mold. It would be quicker and almost as light to use wood strips as a core.

Gary

[Willallison]

T-McG: modelled in MultisurfLT

Gary: hmm....was hoping to build it over frames rather than in a mold - can't you do that with foam core in the same way as you would with timber strips?
A quick look at the Gaussian surface curvature shows that the surface is about as undevelopable as you can get so perhaps a little more work needs to be done there!! Obviously, the more developable my panels, the easier it will be to build.

[mmd]

Beautiful hull, Will. If I may make just the smallest of negative comments (having designed a similar boat for stitch & glue ply construction and being taken to task by a veteran rower for the following, I can't resist passing his comments along), you may wish to alter the lines aft so that the transom is carried clear of the waterline at normal loading. The immersed transom will add drag, shortening the "glide" between strokes. In other words, the rate at which the boat will lose headway as you recover your oars for the next stroke will increase if the transom is submerged. If the submerged waterlines are effectively that of a slender double-ender (or canoe body) the boat will carry her speed better between strokes, thus increasing the distance made good per stroke. Keeping the general form and carrying the transom above LWL will be argued by purists as wasteful because it is extra hull weight that you are carrying around, but these are also probably the people who shave off their body hair to reduce wind turbulence. Keeping the transom will aid the form stability of the hull when heeled. I'd suggest shaping the hull so that the stability effects of the transom begins to come into play at about 5 to 7 degrees of heel.

As for construction, keeping foam core strips as thin as I suspect that you want to build fair will be a task. If experience with the process is your goal, have at it; if light weight and stiffness is the target, I'd strip-build it with wood & epoxy. But then, there are as many arguments about hull materials and techniques as there are boatbuilders, I think. (Wait! Did I hear a voice in the distance calling, " Carbon Fibre!" ? <lol>)

[gonzo]

If you build with foam and fiberglass the weight will be rather high on a boat that size. The fairing compound alone will add twenty pouds. Foam/fiberglass panels can be made light, but they'll have no puncture resistance. You say the boat will get rough service, so it will delaminate after pounding on shore a few times. A solid fiberglass hull is more appropiate for the use you describe. Also, if the chine can have a sharper edge, it could easily be built of plywood.

[Tom_McGuinness]

mmd said:

"taken to task by a veteran rower for the following, I can't resist passing his comments along), you may wish to alter the lines aft so that the transom is carried clear of the waterline at normal loading. The immersed transom will add drag, shortening the "glide" between strokes. In other words, the rate at which the boat will lose headway as you recover your oars for the next stroke will increase if the transom is submerged. If the submerged waterlines are effectively that of a slender double-ender (or canoe body) the boat will carry her speed better between strokes, thus increasing the distance made good per stroke."

Agreed. This is precisely the kind of change we were considering.

Do you have any suggestions for reducing/minimizing this hull's turning resistance, assuming there will be a daggerboard somewhere amidships. Am trying to strike a balance between speed and maneuverability. (I'm not an NA, so please forgive my ignorance)

Regards,

Tom

[Willallison]

Thanks for all your comments guys - all are welcome.

I've never built using foam core (hell - I've never built period!), so all your advice is keenly sought. I was thinking of using corecell or similar, with relatively thin glass skins.

Gonzo - I take your point about durability - though I didn't mean that the boat was likely to be bounced off the rocks, more that the loading and unloading process will no doubt result in the occaisional bump....). I suppose that in order to make the skins strong enough to withstand this stuff, the resultant weight will be too high..? Total target weight for the finished boat (less crew) I guess would be 40kg or less.....
Regardless of materials, in order to simplify construction, I could look at hardening the chines up and perhaps flattening some of the surfaces, but I thought it might increase the drag too much......

mmd & T-McG - I've lifted the transom as you suggested - in looking at the lines I too thought the submerged transom might be a problem. (Tom, I wasn't planning on having any kind of keel / daggerboard)

[duluthboats]

Are you using a sliding seat rig,if so how much will it weight? If you use a fixed seat the finished boat in wood core should be under 40kg.

Gary

[Tom_McGuinness]

"(Tom, I wasn't planning on having any kind of keel / daggerboard)"

Sorry for the confusion...my fault. (g)

The daggerboard is something we're planning for our tailboat, for which we plan to begin hull design shortly.

Not being a NA myself, I was trying to elicit some pointers/insights on where the biggest bang-for-buck might be in terms of reducing hull turning resistance on a cruising type hull. Being quite experienced with the "operational" side of sailing, I'd like the tailboat hull to pivot sharply about the daggerboard as a sailboat does during tacking....like a Catalina 34 (sweet).

Your hull looks very much like something I'd envisaged, but from purely an intuitive standpoint.

Best regards,

Tom

[Willallison]

I was planning on using a sliding seat - haven't a clue how much they weigh!!

Tom - if I'm worng there are plenty more knowledgeable who can correct me but I think the correct location would be at the LCF - longitudinal centre of floatation..... but unless you're expecting tremendous sideways forces - such as a sail or water skier produce - why would you bother with one at all? Most powerboats turn without difficulty, and installing a daggerboard just increases the possibility of broaching when running down-sea...

I've done a quick model, essentially using the same control points to produce a hard chine version of my shell - it requires some tweaking to smooth out some of the hull contours, but what do you think...

[Willallison]

oops - forgot the pics