Input desired on 20ft row/kite boat design

20ft row/kite boat design

I've been working on this particular boat for over a year. It would be great to get some input on it's design. Actually several years, it started out as a Whitehall inspired row boat.

Design goals:

• 20ft max length
• Disp 650 to 900 lbs
• Dry weight under 300 lbs
• Efficient under human power - oars or pedal/prop
• Fast when under kite - planing
• Easy beaching/shallow draft
• Stitch and glue construction
• Covered sleeping area - forward half decked over
• Self bailing, possibly open transom
• Area of use - Puget Sound/Inside Passage

I've tried to design it such that it should plane well without destroying rowing ability.

Some numbers from the current design;

At 640 lbs displacement;
Draft = 4.8"
Wetted surface = 49 sq ft
Cp = .58
Resistance at 4 Kts = 7.2 lbs
Resistance at 6.2 Kts = 21.0 lbs

At 950 lbs displacement;
Draft = 6.0"
Wetted surface = 58 sq ft
Cp = .63
Resistance at 4 Kts = 8.4 lbs (about 40N)
Resistance at 6.2 Kts = 23.3 lbs
Resistance at 14 Kts = 108 lbs
Resistance at 24 Kts = 215 lbs

Questions:

How can I estimate continuous speed while rowing for an untrained rower?

When fully loaded the transom will drag an inch or two - what are the consequences of this?

What can I do with the topsides to minimize spray?

 

Okay, I'm shamelessly bumping my own thread. I hope that is okay since I think that it has gone unnoticed these last two weeks.

Or maybe my design is so perfect that there is nothing that can be said to improve it?

 

The requirements have changed slightly. I now need to accommodate 3 people so the displacement is up to 1,100 lbs.

I've moved the transom to be completely clear of the waterline and made the chine sharper in hopes of making for good planing.


Disp = 1,120 lbs
Draft = 7.8"
Wetted surface = 61 sq ft
Cp = .64
Resistance at 4 Kts = 8.7 lbs
Resistance at 6.2 Kts = 25.2 lbs
Resistance at 14 Kts = 130 lbs
Resistance at 24 Kts = 256 lbs

 

OK, you asked for comments.

The boat has come a long way from anything resembling a Whitehall. Why have you made it wedge shaped in plan view? The fat transom is unnecessary and only adds weight. Weight estimate of less than 300 pounds is non specific. Shoot for less than 200 pounds which is entirely do able. Your curve of areas have placed the largest section areas behind the mid point. That is more typical of a power boat. A Cp of 0.58 is a bit large for a rowing boat.

If you want the boat to row well do not let the transom drag. If you want it to plane readily, you'll need to move the lowest part of the bottom forward of the mid point so that the run angle is minimized. Reconfigure the bottom so that waterlines are more conventional when in displacement mode.

How are you calculating the drag factors? With some design program or other I presume.

No disrespect intended but I would suggest that you delete this design and start over. You must decide whether the boat is to be primarily for rowing or primarily for powering with a kite or whatever non human propulsion you might use. Designing for both rowing efficiency and planing efficiency is difficult if not impossible. You can strike a reasonable compromise with a far more simple layout. If you design the 20 foot boat to row suitably well you might expect to make progress at 12.5 minutes per mile at moderate exertion levels when no opposing current or wind is impeding your progress.

 

My understanding was that you want a higher CP for a boat that you want to go beyond hull speed. How would making the CP lower be an improvement?

Yes, the Whitehall resemblance vanished quite some time ago. As far as weight, it is an estimate. Colin Angus' Row Cruiser is 210 lbs, same length as mine but a lot less volume. Once bulkheads, sole etc are installed I will be very happy to be under 300 lbs.

The curve of areas closely follows how I expect the weight to be distributed, the CG will be quite far aft - probably two feet aft of center. The wedge shape is because I want a very fine entry while keeping it narrow. With most of the weight in the back I didn't want to make the max beam further forward.

What benefits would there be to moving the max beam forward?

The fat transom is for three reasons. The first is I want the room in the cockpit. Second is I want some extra stability for this narrow boat, and third is that I thought it would plane better that way.

I don't follow. The lowest part is already forward of the mid point.

I'm using FreeShip.

Deleting and starting over does sound a bit harsh. I have started over several times. Each time it turns out fairly similar. It's awfully easy to modify it as is anyhow.

I'm thinking it best to consider it a kite boat. When the wind is not sufficient then it is rowed. I'd like to make it row easily while not destroying it's ability to plane well.

12.5 minutes per mile is just over 4 kts. I'll be pleased if a single rower can maintain over 3 kts.

 

Rowing and powered skiff

This type of boat can be optimized to suit your needs.

Welcome

JS

 

I would not take these boats anywhere near open water, narrow-gutted affairs you'd be frightened to stand up in.

 

W9GFO, what is trying to be said, without being overly rude is, your drawing is lovely, but not well suited to what you'd like it to do and since you don't understand the reasons, most of which are obvious, it's clear your need to place more time in studying the hydrodynamic aspects of design, instead of how to manipulate software. Simply put, these free software packages are wonderful, but unfortunately don't tell you what's most appropriate for your desires. This is where the education aspect comes to play. In other words, you have a pretty picture, the software has faithfully generated what you've asked and provided rudimentary resistance and basic volumetric information, but that's it. Nothing on weights, scantlings, appropriateness of shapes selected (volume distribution), etc.

 

If you read my text regarding Keyhaven Skiff carefully you will see that it is an example of how to design a boat. It describes it step by step.
All the parameters must be calculated and chosen with great care and knowledge.

Just because an image looks like a boat, is it not a boat, it just looks so.

JS

 

Thanks for responding. I am glad there is finally a discussion.

I have a pretty picture. I guess that is a start. I have explained why I made the design decisions that I did, but no one is being specific on what is wrong.

Aside from it being too narrow to stand up in for at least one person, what is so obviously wrong? Please note I am not claiming that nothing is wrong, but why not actually say what is wrong so that I may learn something and thus improve my design?

 

The Keyhaven Skiff looks like a nice boat. I read the article, why does the transom need to be 1/2 the max area? How does it affect the boat when it is something other than that?

 

For each speed there is an optimal ratio between the midship area and the transom area. And it must be selected in relation to the planned optimal velocity. Otherwise, the resistance will become larger than necessary. Furthermore, transom depth is calculated so that the transom is dry at the minimum acceptable speed.

Calculate! Do not guess!

js

 

Your questions are where the study of design fills in the blanks. This is a great place to ask a question or two or even several, but a dissertation on yacht design, so you can understand the elements you've missed, isn't best carried out here. FreeShip is a tool, just like a calculator or slide rule, but the fundamentals about design is knowledge, in fact, an engineer discipline. You can acquire enough information on your own, which will require study or you can take a course.

For example, after a little study, you'll realize that developing a rowboat that can plane off, is exceedingly difficult and has so many shape conflicts, that it becomes all but impossible. Have you even seen a rowboat plane? Even the fastest rowing shells aren't in "plane mode", but are cheating the theoretical, LWL based, hull speed limitations with B/L ratio and weight, much like a multihull does. This doesn't mean you design a set of shapes that couldn't "pop up" or surf on a wave or wake, but it does require some understanding of the way things work. This also doesn't mean you can't develop a boat that is able to get into full plane mode, that also can be rowed. The short answer is you can, but you'll have to kiss off something to get something else and more of the understanding stuff is needed.

 

..and this is a problem for me. I do not know what will be the optimum velocity. There is not a lot of examples of small kite driven planing monohulls to draw from.

 

A kite driven hull form, isn't going to be any different than a sail driven full plane mode hull form. There are a few different approaches (have a look at a Laser, then a Swift Solo, for clues) to sail powered planing shapes, so start there.