HMD 19 Scaling Plans

[SP]

I just got done moving from CO to TN and after a few smaller projects, to help my dad out, I will be starting my build.

http://forums.bateau2.com/viewtopic.php ... lit=HMD%2A

I am seriously giving thought to stretching the design 10% in length only.

I know it will be lots of extra work etc... which is OK, but I have one concern.

How is this going to affect the design with respect to its displacement/Water line and how the boat will float and perform?
There appears to be a good amount of rocker in the hull so will the transom and/or bow be too much out of the water.

Evan, will stretching the design 10% in length only be OK?

[chicagoross]

should float to the same lines - you have a 10% increase in bouyancy, but also a 10% increase in weight. As far as performance, length is a good thing for displaement boats. Square root of Waterline x 1.3 give you the hull speed, say 16' so 4 x 1.3 = 5.2 hull speed for the HMD (mine cruises at 6 mph and will go to 7 if you push it.). increase the length 10% and you should increase your cruising and top speed about .25 mph.

[Evan_Gatehouse]

Hi,

Scaling all dimensions by 10% is OK in theory - see below for why you might not want do however. Stretching length by 10% is much harder to make work - none of the frames, cabin sole etc will fit. All the hard work the designer does to make pieces work will be replaced by you taking dozens of templates from the hull. So it is not recommended.

As far as floating - the boat as shown has a plain hull weight of around 1055 lbs. Let's call it 1100 for round numbers. To float on it's lines it will need 2400 lbs aboard. By the time it's painted, outfited for camp cruising and you have a few people aboard, motor, fuel water, food for a weekend, etc etc you will be closer to 1300 lbs of payload than you think.

If you scale it up, displacement doesn't scale linearly - it's a cubic function. So 10% larger = (1.10)^3 = 1.331 x larger displacement. So to keep it floating on its lines you'll need a displacement of 1.331 x 2400 = 3200 lbs. Big difference. The hull weight will of course increase but the payload probably will not. You're not going to squeeze 4 people aboard (or maybe you are?). So.... the boat will float considerably higher than designed. It might pound a bit more in a chop, and maybe feel a bit more unstable since the side chines won't be immersed - it could teeter back and forth between the two shallow V shaped bottom panels. For this hull I wouldnt be keen to advise it.

[SP]

Thank you for the information.

With respect to scaling in length only.

I thought if you took the hull panels and scaled in length only, say 10%, you could use the frames and transom per plan and they would fit if you spaced them out an extra 10% on the strong back.

Is this not correct?

[Evan_Gatehouse]

It can depend on the boat hull shape. Planing hulls with constant aft sections are easy because the frames don't differ very much as you move fore/aft. Displacement hulls - with frames that vary much more in shape, can be a bit trickier. If you go this way, do not be surprised if the frames don't fit properly. I would suggest you make molds of cheap particleboard with the same shapes as the design frames. If they fit, then cut the frames out of expensive plywood. But do be prepared for a bit of shaping of frames to make it all work.

[SP]

If the frames won't fit, any tips on how to stitch and tape up the hull?

If I don't have the correct frame measurements to build the hull on a strongback or cradle I am wondering how to do it and get a straight hull etc....

I know how to measure out the frames from the stitch and taped hull, but how to get the hull done..........

[Evan_Gatehouse]

I just did a 1D scaling of just the hull panels to see how the frames would fit. This is not the same as scaling the flat panels by the way. The middle frames fit fairly closely but the forward ones do not.

There's no easy way to scale the boat in one dimension only and make it all fit, sorry. Consider a 10% scaling of the whole boat or consider a different slightly bigger boat (P21? - it's not a displacement hull but's a shallow V)

[SP]

Evan, thanks for all the information. I am starting to come to the same conclusion. Either build to plan or pick a different design.

[chicagoross]

SP, I went through these same questions when I built my HMD (which I love)...After about a year or two of intense lobbying, Evan designed the MM21, with about 6 or 8 months worth of constant design input and wishes from the forum. I bought one of the first sets of plans, and after sanely resisting for several years, am now mid-build...

The HMD 21 that I had wanted (max garage-build size and easy trailering) would have been much easier than the MM21 to create, but Evan thought he could do better with the extra two feet, and I think he has. I'm really enjoying seeing all of the curves of the MM21 coming together and it will be very distinctive at the ramp. It is harder and more complicated than the HMD though. One of the amazing HMD deals is that it's got to be the simplest cruiser you could possibly build - it doesn't even have stringers! I built a D4 as an intro to stitch and glue, and honestly the HMD is no harder, just bigger.

Also got to say that the HMD is bigger than you think. We've gone day cruising with 5 adults and two kids, all the gear etc and had great days fishing and snorkeling without getting in each other's way - and the HMD doesn't even notice the load, scuppers still above water. If you are serious about a displacement hull, this is one of the best available. Most out there in this size range don't have as much interior room and don't have self bailing cockpits.

[SP]

The design definitely has a lot of positives. I really like the large amount of storage for such a small boat.

It also appears that the hull can carry the load if needed. Although, I haven't seen a picture of an HMD19 actually on its lines in proper trim. Both pictures of HMDs that I have seen, are very bow high. I know the boats are probably very much empty and on the light side, but I am surprised the bow is so high.

Is it normal for a small displacement boat to trim bow up, even when not loaded down? I would of thought the boat would be level on its lines, just higher out of the water.