Water surface roughness vs hull roughness vs boat speed

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pedroe
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Water surface roughness vs hull roughness vs boat speed

Post by pedroe »

I own a homemade jon boat which the hull is not smooth.
At the time it was fiberglassed with chopped strand mat and finished with a belt sander. Gel coat was applied on top with a brush. The hull smoothness is much worse than orange peel. The boat was made in 7 days.
It has 3.8m, a 20hp 2016 suzuki , and in favor of the tides I have reached 51kmh (31.5mph) gps speed.

The boat rides in a protected lagoon connected to the sea. When the water surface has some lumpes, probably an inch high like picture 1, the boat is quite fast. But when the water surface is perfectly still like picture 2 the boat is significantly slower. My understanding is that the boat rides on top on the water lumps, therefor there is less contact in terms of area between the hull and the water surface thus producing less drag.

The difference is so significant that I always try to choose a path in which the water has some roughness. I can even feel the boat accelerating whenever I transition to rough water surface.

Sometimes the lumps on the water surface are produced by some very light wind, other times it looks to be turbulence from converging channels. Is not the wind direction that is accelerating my boat, since I always feel the boat increasing speed independently of the wind direction.

Does the theory corroborates my observations, or my boat is a special case 8O ?

Let's imagine now that the hull was a invented mold of the water when it has some lumps. Riding on perfectly smooth water on such a rough hull would have the same effect of riding a perfectly smooth hull on rough surface water.

This would make me conclude that a not so perfect hull is more efficient on a planing boat, which might be a nonsense.

My I ask for some thoughts on this matter?


Picture 1
passeio_birdwatching_ria_formosa.jpg

Picture 2
RiaFormosaQtaLagoPortugal.jpeg
RiaFormosaQtaLagoPortugal.jpeg (36.82 KiB) Viewed 790 times




This is my boat. Not sure if the hull roughness is perceptible in the photo.
FB_IMG_1674285645211.jpg

fallguy1000
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Re: Water surface roughness vs hull roughness vs boat speed

Post by fallguy1000 »

Unfortunately, your theory has been well proven to be nonsense, but..

There are aspects of it that may have possibilities.

The fastest a planing hull can travel is when the least amount of water is in contact with the hull and this is the key to understanding the phenomena. In most boats this occurs when the highest amount of power is given and the best results are generally on the flattest, non-wind sea states. However, the jon boat has a lot of surface area in contact with the sea when lightly powered.

And so, if there are small surface effects, it is possible the boat is riding on top of them and getting a bit of air under the hull which is creating some lift. A bit of air has less drag than water and so the same boat, if she can get a bit of air under might have the hull acting like a sail. This is, of course, a fine line. If you have massive headwinds; they will slow the boat. And so, you are seeking that light rough water to reduce the drag of your crappy bottom by getting a bit of lift.

This is sort of the elemental concept of foiling, that air has far less drag than water..

So, despite the general idea being nonsense, the phenomena is actually showing the effect of surface drag of a lousy bottom. Additionally, if you have a bit of hook due to the glasswork; the wind or crest riding may reduce the negative effects of that and you really ought to check for it with a perfectly straight 8' timber or straightedge.

If you erase any hook and poor surface; lumpy water may still be your friend which is largely a testimony to being slighlty under ideal power or prop design.

And prop slip is also surely a factor. A prop would be slipping more with a rough surface with more hull in the water. So your prop efficiency is poor. If you get a small bit of lift from air on the sea top; the prop would slip less, except in headwinds, of course. Here is a way to calc prop slip for any boat. I plan to calc mine this coming season. https://teaguecustommarine.com/teaguebl ... ller-slip/

But finishing that bottom will pay off in fuel economy; that is the bigger story.
My boat build is here -------->

viewtopic.php?f=12&t=62495

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Jaysen
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Re: Water surface roughness vs hull roughness vs boat speed

Post by Jaysen »

An actual architect needs to chime in, but I’m wondering if a little step chine stile aeration is occurring which overcomes the obvious ill effects of the finish. I’d put money on a couple hours with 80grit to make it fair and orange peel equal, you will up that top speed end even more.

If you’re up for some fun, maybe skip the “sand it” step, give it a nice muriatic acid (on/off) wipe down, soda blast then just drop some fresh gel on top. Local guy did that 19’ work boat here. Seems to have worked well. No idea how long it will last but sure looked nice at the ramp.
My already completed 'Lil Bit'. A Martens Goosen V12 set up to sail me to the fishing holes.
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Jaysen wrote: Mon Apr 29, 2019 3:44 pm I tried to say something but God thought I was wrong and filled my mouth with saltwater. I kept my pie hole shut after that.

TomW1
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Re: Water surface roughness vs hull roughness vs boat speed

Post by TomW1 »

Hey guys I do not know where Pedroe is from as he did not give his location. But you're getting pretty fancy. First fallguy the prop probably came with the motor and for a 20HP there are few choices. The best thing he could do is sand down the bottom smooth and then reapply the bottom coatings. The one thing we all know is that we want a smooth bottom. Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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BarraMan
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Re: Water surface roughness vs hull roughness vs boat speed

Post by BarraMan »

The one thing we all know is that we want a smooth bottom
....... and not just on boats! 8O :lol:

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Re: Water surface roughness vs hull roughness vs boat speed

Post by Dan_Smullen »

BarraMan wrote: Sun Jan 22, 2023 1:01 am
The one thing we all know is that we want a smooth bottom
....... and not just on boats! 8O :lol:
Thank you, BarraMan! Some benefit to a bulbous bottom too, although difficult to measure with science

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Re: Water surface roughness vs hull roughness vs boat speed

Post by Jaysen »

I mean… length at the water line and all.
My already completed 'Lil Bit'. A Martens Goosen V12 set up to sail me to the fishing holes.
Currently working on making a Helms 24 our coastal cruiser.
“Mark Twain/Samuel Clemens” wrote:Eat a live frog first thing in the morning and nothing worse will happen to you the rest of the day.
Jaysen wrote: Mon Apr 29, 2019 3:44 pm I tried to say something but God thought I was wrong and filled my mouth with saltwater. I kept my pie hole shut after that.

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OrangeQuest
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Re: Water surface roughness vs hull roughness vs boat speed

Post by OrangeQuest »

What Fallguy suggested is the correct answer. Rough bottom and smooth water are higher resistance than the rough bottom and a lot of small air pockets trapped under the planning hull in ripples. The rough bottom may help trap more air.
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pedroe
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Re: Water surface roughness vs hull roughness vs boat speed

Post by pedroe »

OrangeQuest wrote: Sun Jan 22, 2023 2:58 pm The rough bottom may help trap more air.
I don't know if the bottom was smooth the boat would perform better or not. I can only say that in the present conditions, ripples in the water are extremely beneficial

It reminded me golf balls. I don't know if there is a similarities with this case.
Golf-ball-surface-Figure-c-shows-the-pre-agglomerated-element-sizes-the-actual.png
Golf-ball-surface-Figure-c-shows-the-pre-agglomerated-element-sizes-the-actual.png (86.78 KiB) Viewed 658 times

fallguy1000
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Re: Water surface roughness vs hull roughness vs boat speed

Post by fallguy1000 »

The boat would perform better smooth.

The aerodyamic qualities of dimpled spheres is something entirely different.

Do the prop slip calcs.
My boat build is here -------->

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