# Hull Resistance

Discussion in 'Propulsion' started by Javaid Hosany, Apr 18, 2008.

1. Joined: Apr 2008
Posts: 20
Likes: 0, Points: 0, Legacy Rep: 10
Location: Mauritius

### Javaid HosanyJunior Member

Is there any formulae to calculate the hull resistance for small boats (14" to 20")long, which will be used in the lagoon? I have to calculate the thrust needed for these range of boat.
Thanks

2. ### Guest625101138Previous Member

Yes there is. You would need to provide more detail on the shape and desired speed to get the right formula though.

Rick W.

3. Joined: Apr 2008
Posts: 20
Likes: 0, Points: 0, Legacy Rep: 10
Location: Mauritius

### Javaid HosanyJunior Member

Thank you lots for your reply Rick, I'll get the boat within a few weeks, and i'll send you the details and photos as soon as i get it.
Thank you lots.

4. Joined: Sep 2005
Posts: 48
Likes: 5, Points: 8, Legacy Rep: 80
Location: Calcutta,India

### Bijit SarkarNaval Architect

Javaid,
Your boat may be of displacement type, meaning at ful speed the weight is still supported by buyancy, or planing, meaning the weight is supported by the planing action and most of the boat is out of water.
As Rick said, once you give the length, bradth, draught, and the desired speed, one can, with some accuracy , predict a power requirement for your boat.
Either way, 14 to 20" would be too small I presume you mean 14-20'.
I have an FRP boat of 18' length, which starts planing at 17 knots and goes t a max speed of 23.5 knots.

5. Joined: Apr 2008
Posts: 13
Likes: 0, Points: 0, Legacy Rep: 10
Location: Brazil

### skipper_rjJunior Member

6. Joined: Apr 2008
Posts: 20
Likes: 0, Points: 0, Legacy Rep: 10
Location: Mauritius

### Javaid HosanyJunior Member

Thanks Bijit Sarkar, I'm still looking for the best boat available here. As soon as i get the boat i'll contact u.
Thanks

7. Joined: Apr 2008
Posts: 20
Likes: 0, Points: 0, Legacy Rep: 10
Location: Mauritius

### Javaid HosanyJunior Member

Thanks lots skipper_rj, very helpful site.

8. Joined: Dec 2001
Posts: 2,474
Likes: 116, Points: 63, Legacy Rep: 1728
Location: Oriental, NC

### tom28571Senior Member

This seems like the wrong way round. Finding the "best" boat should follow knowledge of what the requirements are. What constitutes "best" depends on the intended use and goals you wish it to satisfy.

9. Joined: Sep 2005
Posts: 48
Likes: 5, Points: 8, Legacy Rep: 80
Location: Calcutta,India

### Bijit SarkarNaval Architect

Dear Javaid,
Tom is very right. If you have got the boat already, there's little , if anything , you can do except buying a more and more powerful engine.

I suggest you look at the alternatives available to you and check what speed you are getting and if that is serving your purpose. I am sure you will get a lot of good advice from all our members.

Amongst things you look for the primary condition should be the condition of the hull. If you are buying FRP, look for striation - meaning layer separation in the laminates. Its easy to make out by light tapping with a mallet and listening for a hollow sound.

If steel, get a gauging done .

Do a speed check. Buy or borrow a gps and that should give you a fairly accurate speed .

Do a few speed turns ( carefully please) to judge if the boat has a tendency of side slippage .

Above all, the boat should feel right to you. You should feel that she has been waiting for just you to come across.

All the best.

10. Joined: Mar 2004
Posts: 27
Likes: 0, Points: 0, Legacy Rep: 10
Location: Australia

### kenwstrJunior Member

Hi,

Similar to this, I am looking to power a couple of sailing dinghies. Prefer electric though open to consider other options. The reason being negotiating windless river channel to make for more open water. Given the tide is not too severe, I only need to power within displacement mode speeds. This should keep costs and mechanical stress down (one of the boats is not designed to carry a motor). Also one of the boats could double for trolling and fishing, again slow speed. However it is somewhat unusual to power this type of boat at such slow speeds.

Given that electric motors are rated by thrust, I only need a ball park drag figure for the 2 boats. How can I relate the length, displacement and speed to the required motor?

14' LWL, 240 kg displacement fully rigged and crewed, 5 kts (very efficient performance sailing hull)

3 people in a 9' tender 3-4 kts

Is there a simple way to work this out?

Regards,
Ken

11. ### Guest625101138Previous Member

Rated thrust for a trolling motor has no meaning once you want to make progress. It only has meaning for a tug of war. It is power that matters and you will need maybe 300W to get the longer boat to 5kts.

There is no simple way of determining the drag. You will get the most accurate answer using Michlet but this requires a bit of effort to apply. If you post a picture of the hull that allows it to be drawn then you could get a more accurate answer.

However does it matter - you have limited options with the trolling motors so why not just get one and try it out. The wave drag will be kicking in at 5kts on the 14ft boat so just dropping your target to say 4kts will pull power back to around 100W. You will get a lot more distance from your batteries.

The 3 to 4kts for the 9ft boat is also realistic. A lot will depend on the transom drag but this can be reduced by shifting weight forward. Again 3kts will be much more economical.

Rick W.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.