Half-sized model

I have a 32 ft LWL planing boat that I want to explore converting to a displacement hull. I'd like to explore the effects of adding on various bow and stern extensions. (I know, I know - I don't need to be told about the foolishness of such a venture. My sanity is not a topic of discussion for this thread...

The existing hull lines are quite simple and for relatively low cost I could build a quick & dirty half-sized model of it and add ballast until it reaches the appropriate waterline. I would then power it with an outboard. Once I had run some sea trials to calibrate its performance to the full-sized model, I would tack on the various hull extension mock-ups and run further sea trials to see how the speed and efficiency performance changed.

I could accomplish some of this with computer models, but there's really no replacement for performance testing to gather insights. And my computer modeling tools, skills, and interest are limited. I could hire a NA to perform the modeling for me (and I still may), but for a lot less money and a bit of my time (and building boats qualifies as recreational time for me) I could build this test hull and the various flavor extension add-ons.

I'm not looking to predict exact performance numbers for the full-sized boat, but rather, get a sense of the relative performance improvements (or not) that I could expect if I applied those mods to the big boat.

Is my logic flawed here? And are there any major corrections I should make to any information I glean before I extrapolate it to the full-sized model?

 

Save yourself some money and building effort and just ask.

Converting a planing craft to displacement mode, makes a lousy displacement speed boat. It'll drag it butt, wallow around, steer like crap, as the hull form is not conducive to displacement mode efficiency. You could put a bulbous bow on, but it's not going to help much, if at all. You could add something to the stern, but this will just be a bandaid to a poorly shaped hull for the wave train it'll create at these speeds.

In short, there are huge differences in the hull forms employed in full plane mode and displacement speed vessels. No amount of noses and butt extensions will really do much to help these differences. In other words, you might gain a few percent in efficiency over the stock full plane at displacement speed efficiency levels, but will still be woefully below that of a craft designed for displacement speeds.

 

Thanks PAR. You speak the truth. Almost...

What I neglected to include in my original post is that the hull I'm modeling is one hull from a planing catamaran. The L/B ratio is fairly large, and will get larger with the extensions I'm looking at. Current LWL is 32 ft and the beam is 5 ft. My extensions could conceivably increase that to 45/5 ft. I expect handling to be very acceptable.

I'm currently running at displacement speeds now and the handling and performance are pretty good, though I can't run much above 8.5 kts before before my fuel economy plummets.

I think that I can convert this boat to a decent displacement cat with my modifications. Certainly not as efficient as a purpose-built displacement cat, but perhaps good enough for my needs. If I can achieve anything near 15 knts getting 2 NM/gallon, I would declare this a total success. Even 12-13 knts at that efficiency would be acceptable. That's a 50-80% performance improvement over what I'm getting now at displacement speeds.

The hull is aluminum and has simple lines, so adding extentions is entirely feasible from a fabrication standpoint.

I happen to like the current boat very much as configured as it almost perfectly suits my needs. There are no production displacement cats on the market that would meet my needs nearly as well, and I think I can modify this boat for far less than I can buy a new custom boat.

Now, back to the model. Any thoughts regarding the half-sized model?

I think I can build a rough plywood test hull in about a day. The extensions might take another day. I'm a pretty fast builder, and this model will be quick & dirty, only meant to last the duration of my testing. 16 ft long by 2.5 ft wide, with very straight lines. The investment in time and materials will be pretty low, and the insights could be pretty profound. If I proceed with this I will certainly share my findings here.

Any suggestions on how to get the most valuable data out of my tests would be appreciated.

 

Beware the conversion factors when halving the size of the hulls. Volume is a third power function and speed something in the neighborhood of a cubic function. If the big boat displaces X pounds then the half sized one will displace only one eighth X. Scalewise the small boat would be tested at one fourth the speed of the big one. If, say the big one was calculated at 16MPH then the half sized one would test at 16/4 = 4 MPH.

That is all very well but it ain"t gonna work out quite so neatly as the math would have you believe. Ball park idea? Maybe.

I'd forget the half size boat and just go ahead and do the mods to the big boat, and call it good.

Are you figuring to make the hulls double enders? Or do you anticipate transoms? If you intend to have transoms you'll need to get them up a bit above the waterline. But you already knew that I expect.

 

Thanks messabout, that's kind of what I figured.

I have some ideas for different transom and bow shapes and I'm not sure which ones will work best. With my half-size, I can swap out the different shapes (I'll probably just attach then with drywall screws, or something similarly crude) and then see how they do relative to each other.

I figure if my modifications on my model increase its speed by, say, 50%, then I should be reasonably confident that I would see a similar increase in the big boat. The speeds between big and half-pint will be different, but the relative changes should be generally similar. If this was a 1/10th model then I'd be much more worried that the physics wouldn't carry over.

And more importantly, if I add on my hull extensions and see little or no improvement, then I've saved myself the large expense of converting the big boat for minimal benefit. Doing the half-pint is a way to minimize my risk. But maybe I'm missing something here that invalidates my logic.

My thinking is that, since this is a displacement (semi-displacement perhaps) boat, I'd run my half-pint up until I reach hull speed, noting the throttle setting/RPM of the outboard, and using a GPS to document speed. Then I'd add my extensions and run it up until I reach the new hull speed, and document my outboard throttle/RPM. It would be crude, but should give me a pretty good idea of the relative changes. If the hull speed increases by 10% but I need to use 100% more power to get there, that's not a good tradeoff.

As far as transoms, I plan to explore that with my extensions. A displacement catamaran can clip along at a pretty good speed, so maybe a shallow immersed transom with a hard break makes sense. Maybe not.

 

I'm sorry I have to contradict messabout. You're interested in the speed characteristics in displacement mode, so the speed you should use is scaled with the square root of the length. At half scale you should test at 10.6 knots if the speed of interest is 15 knots.

If you have scaled displacement with the cube of the length and speed with the square root, the propulsion power should scale with the power of 3.5 if I remember correctly.

I agree that for us amateurs a scale model is useful to gain understanding. When doing my own experiments I have always calculated/simulated 100% correct so far, and practical tests have only confirmed the calculations. The problems have been all the aspects I didn't think of. Scale model testing might help to reveal those. I also enjoy messing around with boats so I don't mind spending time and money on scale models as long as I learn something.

Erik

 

Erik; you are quite correct. I dropped the ball with the velocity comparisons. (result of the rum toddies maybe) Equivalent S/L or Froude numbers would be the correct way to do the calcs. My apologies to Deering.

The displacement ratio bit is still valid and must be given more than casual consideration because of the eight to one factor.

 

Thanks Erik and Messabout,

As far as displacement goes, my plan is to replicate the hull in all dimensions at half-scale. For the testing I would weight it down to the half-scale waterline. The fullsize draft is 27.5 inches, so I would weight the half-scale to a 13.75 inches draft.

By doing so, the length, beam, and draft are all 1/2 so the displacement should be 1/2 x 1/2 x 1/2 = 1/8, or the cube root. Is that thinking valid?

Erik, I'm trying to make sure I understand the 3.5 bit about the power. The big boat reached displacement hull speed at around 8.5 kts using about 175 hp. The 3.5 power of 1/2 is .09 so the predicted hp is .09 x 175 = 15.5 hp. And that would be needed to achieve 8.5 kts? Or 8.5 * 1/4 = 2.125 kts?

Thanks again. Most enlightening.