Corsair 13 Power version tested

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gk108
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Post by gk108 »

proman_11 wrote:
So does everyone disagree with my idea to get a bigger engine.
Yeah, it's a bad idea. Changing to a bigger motor gives you two things, more weight and more push (thrust). Obviously, more weight on the stern will never make the bow go down. The direction of all that thrust is going to be pretty much exactly in line with the prop shaft. The trouble is that the prop shaft is already pointed toward the sky and pushing harder toward the sky won't make the bow go down.

These are simplified explanations for sure, but there are bookshelves full of information on why boats are shaped the way they are. Most of it is about displacement hulls. There seems to be a zillion different concepts of what makes the best sailboat hull. Planing hulls don't get as much discussion because they must have a flat surface on the back end of the bottom in the fore and aft axis. About all that's left to talk about after that is how much V the bottom may or may not have.

Trim tabs won't help either. In order for them to have the force necessary, they would create too much drag. Same goes for doel-fins on the outboard.

There is only one way to make that hull drop at the bow while motoring:
Throttle BACK
It's all about those laws of physics. You just can't break them. Even if you try, one or another of those laws will break you first. :D
CC, D15, V10

proman_11
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Post by proman_11 »

Hi everybody. Well, I took the boat out today, along with two other friends. I took some short video clips as well on my mobile im just working out how to put them on the internet. complicated because mobile phones have a weird format.
The boat worked good today, I was very happy with it. We spent about 4 hours on the water trying to get her to plane. Im not 100% sure, but in attempting to make her plane, I left my friends on shore and took her out by my self. From a stand still I slowly raised the throttle and the bow began to raise up as usual. But as soon as I hit full throttle the bow came down a fraction, problebly about half of what it had gone up. The boat seemed to be planing.
She held up really well through the chop. At first I was VERY worried about her breaking apart especially with the hard hits she was taken over the witecaps on the river today. The only visible damage out of today was a few hair line cracks in the filet joints where the anchor well sits. Other than that everything else was solid. Im even thinking of calling her ICEBREAKER. or Dicebreaker for a more symbolic meaning.

JAcquesmm, I know your the designer and I take your comments very seriously. But I am confused. In a previous topic "Alternator in 15hp Mercury" you said
"1. I agree that you don't need an alternator to run those things.
2. About the CR13 not being a power boat, I stand by that BUT IF there is one sail boat that can plane with a motor, it is the CR13. There is no rocker in the aft, we designed the boat to plane easily on sail and to be used as a fast dinghy with outboard. She also has an unusually wide transom. This boat is an exception to the rule, you can add power and plane.
My comment reflected worries about the structure but it looks like you reinforced the transom etc. It will work."
Jacquesmm, is there any possiblility that this hull could take on more horsepower? Im thinking of buying a 30 hp mercury or adding on a 15 to make the boat a novelty boat. What are the risks involved in this? I really believe the hull is underpowered. Please take into consideration that I have added ALOT of weight to the boat with the flat floor, excessive fiberglassing and reinforcements. (Also because i am an amature i wasted alot of epoxy...)

To ks8 you said "Maybe soon, as you refine exactly what you want in a *dream boat*, or just a next step, the group here can help you with a model that will better do what you are hoping it to do. " Im just wondering if what you mean by that? Can a boat be designed just for me? or for a group? I really love the bostonwhaler outrage, the 30ft version, but I want to build a 20-25 ft version. I really want to start over with a boat, I would love to make it a perfect boat haha (seems weird ha?).

Any ways thanks for all your comments. hope you comment more!

Charlie
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Post by Charlie »

proman,
You have a displacement hull. Like all displacement hulls it will get up to a characteristic displacement speed with very little horsepower. But, if you try to go over displacement speed the boat will try to climb it's bow wave but never quite get up on top. You are then using your horse power to keep the boat on that hill of water. To sustain your boat there you will have to run the guts out of your motor. You may even get a little over hull speed but it will take a lot of rpm's and a lot of gas. I don't know if it's accurate but I've heard that to go one mile an hour over hull speed takes a doubling of horsepower. And the next mile per hour another doubling. Even if this idea is hot air you are in a situation where a bigger motor is not going to solve your basic problem. What the bigger motor will do is allow you to cruise around with your bow higher in the air than with a smaller motor.
Displacement speed= square root of the water line times ~1.4
No one has yet found away to get completely around the limitations of the above equation. But, some still try.
Good luck.

Jerry-rigged
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Post by Jerry-rigged »

Looking at the profile shot of your hull up-side down, there is not much rocker in the hull, but it is there. By sailboat standards, there is not much, but by powerboat standards, it is still way too much. Ask some of the builders around here that have accedently added 1/2" of rocker or hook to an otherwise flat planing hull - the results are always more than the builder imagened.

D your self a favor and buy a few books on boat design. Even of you are not a big reader, with your intrest in boats, I think you would enjoy "The Nature Of Boats". It is a good place to start - it mixes boat design with good story telling to keep it from getting tough to keep reading.

Jerry
Fishing from a paddle boat...

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Post by jacquesmm »

I have written on the message board that she can be "used as a fast dinghy with outboard" but that doesn't mean more than 5 or 6 HP and doesn't mean that she is a true power boat. You can clearly see that this is a sailboat.
The CR13 has almost no rocker aft and is a planing sailboat. Given sufficient wind, the boat will plane or more exactly surf occasionally.
What we call planing in sailboats is not exactly the same thing than in power boats. It is the same type of lift but except in extreme conditions, sails will not keep a boat on plane for long.
With that almost straight run aft, the CR13 can also plane under power but not like a true planing power boat.
With a small outboard, a light crew well placed, the boat will go above hull speed but never very fast compared to true planing boats. A small outboard is between 3 and 6 HP but even on the study plans, I write a "very small outboard":
http://www.boatplans-online.com/studypl ... _study.htm
I owned a boat in that style and used her as a dinghy. With a 6 HP, I was going between 10 and 12 mph without much fuss. This is well above hull speed but that's about all the boat will do and only if kept very light.

You drifted far from the original design, sorry. It is clear that the design is a sailboat, not a powerboat with center console.
The added weight, the heavy engine are a problem: the boat is not designed for that, sorry.
I would keep her as a semi-displacement hull. You probably go at around 12 to 15 mph and that's all you will get out of it.

The FS12 and FS14 are true planing hulls. With the HP I specify, they will easily go above 20 mph, probably up to 25 and that is fast for that size boat. They will go faster with more HP, the hull shape can take it but I don't think it is safe. The USCG calculations agree with that opinion.
Jacques Mertens - Designer
http://boatbuildercentral.com

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Post by jacquesmm »

I have written on the message board that she can be "used as a fast dinghy with outboard" but that doesn't mean more than 5 or 6 HP and doesn't mean that she is a true power boat. You can clearly see that this is a sailboat.
The CR13 has almost no rocker aft and is a planing sailboat. Given sufficient wind, the boat will plane or more exactly surf occasionally.
What we call planing in sailboats is not exactly the same thing than in power boats. It is the same type of lift but except in extreme conditions, sails will not keep a boat on plane for long.
With that almost straight run aft, the CR13 can also plane under power but not like a true planing power boat.
With a small outboard, a light crew well placed, the boat will go above hull speed but never very fast compared to true planing boats. A small outboard is between 3 and 6 HP but even on the study plans, I write a "very small outboard":
http://www.boatplans-online.com/studypl ... _study.htm
I owned a boat in that style and used her as a dinghy. With a 6 HP, I was going between 10 and 12 mph without much fuss. This is well above hull speed but that's about all the boat will do and only if kept very light.

You drifted far from the original design, sorry. It is clear that the design is a sailboat, not a powerboat with center console.
The added weight, the heavy engine are a problem: the boat is not designed for that, sorry.
I would keep her as a semi-displacement hull. You probably go at around 12 to 15 mph and that's all you will get out of it.

The FS12 and FS14 are true planing hulls. With the HP I specify, they will easily go above 20 mph, probably up to 25 and that is fast for that size boat. They will go faster with more HP, the hull shape can take it but I don't think it is safe. The USCG calculations agree with that opinion.
Jacques Mertens - Designer
http://boatbuildercentral.com

proman_11
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Post by proman_11 »

Hahah thanks alot for all the information. Ive finally given in, im not going to upgrade the engine. Im just going to finish her off, use her for calm days of fishing in the river. Ive decided to start a CS23 or CS25. Mum agrees, dad doesnt. Thanks alot again. Have fun boat building guys!

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Post by jacquesmm »

You gained a lot of experience building that boat. The pictures show quality work. Use that experience to build a real power boat now and during the building, take a day off from time to time on your hybrid CR13. It's still a very usable boat.
Jacques Mertens - Designer
http://boatbuildercentral.com

Dougster
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Post by Dougster »

The Dougster's real impressed with what you built. Good grief, what a first effort! If you keep it goin' on, you're gonna have it goin' on!

No foolin', I like the boat Dougster

ks8
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Post by ks8 »

A few boats here have been designed based on particulars that several builders were looking for in a boat. I think when Jacques has enough committed builders for the plans for a new design, he designs according to what they are looking for. It can't hurt for you to join some discussions now and then about any ideal you are looking for. But I had been thinking that we could help first with existing designs to better meet your ideals for the next boat. Talk it up and see what happens. :)

CS23? So let's start. Talk about the particulars. What made you look at the CS23? List some guidelines that you think bring you to the CS23... 8)

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