designing a fast rowboat

- then checkout http://www.kayakforum.com/cgi-bin/Building/index.cgi/

 

Nick's work is awesome stuff. I've had his boat **** bookmarked for ages.

AFAIK Delftship hydros are accurate. The free version is limited for functionality, but I'd trust it for what it does. It was originally developed for serious professional work. Check the list of references.

I have been thinking about the concave sections on GB's. Two things spring to mind. First is that enthusiasts will invariably come up with all sorts of rationalisations for anything, regardless of whether they're accurate or not.

Second thing is that GB's had to operate with a wide range of loading, from 150 lbs or so with just one guide, up to a quarter ton or more with a guide and a sport and some gear and a dead deer and whatever else.


We know the hard blige above the usual waterline adds reserve stability and bouyancy, which is much needed when heavily loaded. However, cutting that away into a concave might reduce waterline beam when lightly loaded (guide only) so could make the boat easier to row solo at the cost of some increase in resistance when more heavily loaded.*

This seems logical, and may even be true. I'm not sure it's useful for going around Cape Ann though.

I have thought of scaling the GB to 17 or 18 and seeing how the resistance pans out. Just out of interest, one of the Godzilla/Michlet hulls so far came in with 83% of the resistance of the 16 foot Grant GB at 5.2 knots, and much the same initial stability. That was on a 19 foot waterline with a 26" BWL, and a bit of rocker fore and aft. Displacement 294 lbs in salt.


*ETA: Had another look at Virginia's lines. The hollow amidships is too slight and too low down to have any effect on BWL, even when lightly loaded. My 2c: they did it because it looked cool, then thought up "reasons why it would work".

 

Here ya go: original 16 foot guideboat compared with the same boat scaled to 18 feet.

DWL is adjusted to give the 18 footer 10 lbs more displacement, just to account for extra structure. Difference in resistance at race speed is just over 10%. They're even at about 4.5 knots.

 

The hollow appears to be only the first plank so I wonder if it was to make it easier to get enough twist to meet the stem vertically?

 

That is interesting that it isn't until 4.5kts that the longer hull form's resistance increases at a smaller rate than the shorter boat. I guess this is because most of the resistance is wave drag at these speeds >4.5kt?

You refer to "one of the Godzilla/MICHLET hulls at 83%...."

Is one of these the Grant GB you posted or a variation of it that we have not seen yet? How different does the midship section look?

 

Possible. On the other hand, I'm fairly sure that if he had wanted the extra twist he would have done it.

Another point is that Virginia has a lot of volume forced into the ends higher up. The waterlines get quite a shoulder to them. It's almost like someone had put a container ship hull on top of a slick rowboat bottom (*waits for screams from guideboat fans*).

Apparently the reason Grant changed his stem profile from the "canoe look" to plumb was that the ends were looking too boxy up high, due to the amount of volume he wanted there. My current guess is that the flare required to blend this shape required some hollow sections near the ends, and he carried it through just because it seemed right. Hard to tell without having him around to ask.

 

Funnily enough, no. It's just that the wave drag start to be significant enough. The figures are wave drag 15% of total for the 16 footer, and 11.5% of total for the 18 footer, at 4.5 knots.

At 5.2 knots it's 34% for the 16 and 21% for the 18.

The midship section looks wrong. It does well elsewhere, but by default Michlet/Godzilla assumes vertical topsides above the DWL (like commercial/naval ships). I'm actually going to change the way I set the files up, after re-reading the manual yesterday (RTFM FTW ). I found a trick I can apply to get it to spit out midship sections that are more realistic for recreational rowboats.

And no, this is one you haven't seen. If you are going for 18 feet LOA, there's not much point drafting a 19 foot LWL option.

The thing is that Godzilla wont necessarily give you what you want. It will only give you what it can find. Its main value is in giving ideas that may not have occurred to you otherwise, and in giving some figures that can be compared to other options. It's not a magic wand that will do all the work for you. Leo is quite up front about this in his manual for the app.

The other thing about it is that is gives best results when unrestricted. If you put a length limit into it, it seems to give a worse result even if the length limit you put in is the same as what it previously found to be best. There seem to be some quirks about how the algorithm approaches limits.

Anyway, I was trying setting limits in non-obvious places (wetted surface, prismatic, etc) to see what it spat out. It's not hard to do this, because the app just merrily chugs away once you kick start it. The hull mentioned was just one that turned up.

 

The St Lawrence River Skiffs also have the hollow garboards, but I think it was something more to do with keeping the skiffs narrow on the water line when lightly loaded.

That container ship in plan view look is exactly one of the features that I've never understood....but it must also be what carries through below the waterline and helps give the GB its high prismatic.

The SLRS Annie has a lot of hollow as well. Not the Savo.

Look how different these sections are.


Annie_MSsection by Clint Chase Boatbuilder, on Flickr


Savo_MSsection by Clint Chase Boatbuilder, on Flickr

 

Except that it doesn't. It's too slight and too low down to have any beneficial effect on BWL.

Bear in mind that GB builders were trying to maximise useful volume while minimising planking area (weight). Looked at that way, the "container ship" makes sense. Despite what some GB lovers may like to think, GB's aren't some embodiment of hypothetical perfection. They're a series of compromises, just like anything else.

 

Anyone interested in a .fbm file which can be opened in DELFTShip of the lower portion of the hull of the Rushton 16' Guide-Boat "Saranac Laker". It's based on the lines and offsets from O. E. Markula in Rushton and His Times in American Canoeing by Atwood Manley. The construction is not shown so the stem was faired into the bottom board rabbet and is probably too blunt.

The model was created in Rhino, exported as a .wrl file, imported into DELFTship and saved as a .fbm file.

 

Just tried it. Wont import for me. Says it's not a valid Delftship file. I have Delftship 5.06.186.

Are the offsets to the inside or outside of the planking?

 

I just opened the uploaded file in DELFTship 6.13.242. Opens fine and I'm able to run the hydrostatics analysis and resistance analysis.

I downloaded the free version DELFTship today and then it updated to 6.13.242 about an hour ago so I assume it's the latest version.

Offsets and derived surface are to outside of planking.

 

Weird. I'll try updating it later.

 

Attached is a DELFTship coordinate file of the Rushton Guide-Boat lower hull I just exported from DELFTship 6.13.242.

 

That one imports fine for me.