Is Michlet suitable for RC boat?

[mokchie]

Dear all,
I'm designing an RC fishing boat with about 10kg displacement,to estimate its resistance I have spent a lot of time studying Michlet.My initial design is a catamaran with transom stern and a low L/B ratio,then after reading some threads in this forum I'm aware of that transom stern and a low L/B ratio may lead to inaccurate evaluation. So I have changed my design with a high L/B and no transom stern,then at certain speed the outcome is about half of the initial's,and at another bigger speed it has the same value as the initial's.Is that reasonable?Or maybe Michlet is not suitable for a boat so small like mine.
This is my first thread,any suggestion is welcomed.

[BYDE]

hello Mokchie,
Michlet is based on thin-ship theory (if I'm not wrong, I haven't used it), so the hull must be 'thin'. That means a high L/B ratio, doesn't matter the length.
You didn't state your L/B and I didn't fully understand what problems you encountered, especially I found some difficulties in figuring a catamaran with low L/B...

[mokchie]

Originally Posted by BYDE

hello Mokchie,
Michlet is based on thin-ship theory (if I'm not wrong, I haven't used it), so the hull must be 'thin'. That means a high L/B ratio, doesn't matter the length.
You didn't state your L/B and I didn't fully understand what problems you encountered, especially I found some difficulties in figuring a catamaran with low L/B...

I'm sorry, BYDE，I didn't express accurately，by saying “a catamaran with low L/B” I meaned the hulls of the catamaran have a low L/B ratio as 4.6.And I have changed it to be 8.3,then I found the outcome is largely different from the one's before.
But now things have changed, I found I made a mistake when comparing the two boats:they have different displacement volume(one is around 10kg,the other is only around 7.0kg) so they can't be compared directly.And still I have found the offsets I exported from Freeship is wrong.May be after I correct the offsets and set the displacement volume of the two boats a same value their outcomes would not be so much different.
Do you mean a boat of that scale is still suitable to use Michlet? I'm glad to hear that.Because I can't found any other way to estimate the boat's resistance. Now I think I can use Michlet without being hesitating.
Much thanks BYDE.

[Leo Lazauskas]

To estimate skin-friction, Michlet uses either the ITTC line or a variation of Grigson's line. These lines both assume turbulent flow from the bow which might not be applicable to small boats with Rn less than about 10^6.

Good luck!
Leo.

[mokchie]

Originally Posted by Leo Lazauskas

To estimate skin-friction, Michlet uses either the ITTC line or a variation of Grigson's line. These lines both assume turbulent flow from the bow which might not be applicable to small boats with Rn less than about 10^6.

Good luck!
Leo.

Yes,the Reynold number of my boat is about 1.47x10^6, so it works in transition flow regime. But I think I can still use Michlet by modifying its outcome with Prandtl's formula：Cf=0.455/(lg(Rn))^2.58-1700/Rn. I have found the ITTC's answer is much bigger than Prandtl's formula's.
Thanks for your reminding.And your program is an excellent work, I benefit a lot from it,thank you so much!

[BYDE]

I'm sorry, BYDE，I didn't express accurately，by saying “a catamaran with low L/B” I meaned the hulls of the catamaran have a low L/B ratio as 4.6.And I have changed it to be 8.3,then I found the outcome is largely different from the one's before.
But now things have changed, I found I made a mistake when comparing the two boats:they have different displacement volume(one is around 10kg,the other is only around 7.0kg) so they can't be compared directly.And still I have found the offsets I exported from Freeship is wrong.May be after I correct the offsets and set the displacement volume of the two boats a same value their outcomes would not be so much different

it seems like you give yourself a bit too much design freedom

Do you mean a boat of that scale is still suitable to use Michlet? I'm glad to hear that.Because I can't found any other way to estimate the boat's resistance. Now I think I can use Michlet without being hesitating.
Much thanks BYDE.

wait a second, just to be sure: it's a displacement hull, right?

[mokchie]

Originally Posted by BYDE

it seems like you give yourself a bit too much design freedom


wait a second, just to be sure: it's a displacement hull, right?

Yes,it's a displacement hull.
You are right. I think I shouldn't set the ship's displacement volume less than 10kg, because the boat is designed to carry 1.5kg fish bait. If the displacement is too small the boat may trim badly when the bait is unloaded. I have to reconsider it carefully.

[BYDE]

carry a bait, interesting... what are you going to do with the little boat?

you should fix a target weight estimating the weight of every component of the boat. It's an rc model so it should be an easy job.
if you don't have any special requirements, as i guess, i suggest you to pick a systematic series hull and scale it to your weight. quick, and you know the performance of the hull already

[mokchie]

Originally Posted by BYDE

carry a bait, interesting... what are you going to do with the little boat?

you should fix a target weight estimating the weight of every component of the boat. It's an rc model so it should be an easy job.
if you don't have any special requirements, as i guess, i suggest you to pick a systematic series hull and scale it to your weight. quick, and you know the performance of the hull already

The boat is planned to carry a bait to sites far away from the shore, then unload the bait with remote controlling. I have tried to find some systematic series hull, but I doubt that after being scaled it's performance would largely change. And I like to do the design from scratch,it will make me more excited to do it.
I do agree a careful estimate of the weight shoud be made first,in case the displacement is not enough. And it will be very easy. Thanks for your reminding.

[BYDE]

The boat is planned to carry a bait to sites far away from the shore, then unload the bait with remote controlling. I have tried to find some systematic series hull, but I doubt that after being scaled it's performance would largely change. And I like to do the design from scratch,it will make me more excited to do it.
I do agree a careful estimate of the weight shoud be made first,in case the displacement is not enough. And it will be very easy. Thanks for your reminding.

sounds cool

yes it will, in terms of wave resistance, as long as your hull fits in the ranges used during the systematic series experiments.
but of course, if you enjoy doing your own design then i understand.
good luck!