Mast height /performance?

[Gerald]

My plans call for a 13 meter mast. If I increase the mast height and reduce the boom so that I have the same sail area. What will happen to the performance?
Gerald

[ahp]

Your perfarmance to windward improves, provided you can stay right side up.

[mmd]

The sailplan centre of effort will go up and forward, resulting in a change of the hull balance that could induce too much weather or lee helm. Also the forces acting on the mast will have greater leverage, making the boat heel more easily in a puff. The stress on the standing rigging will be greater, meaning possibly heavier rigging and chainplates might be needed. Other than that, not much.

Consult the designer before you go messin' with the rig - any amount he charges you will be worth it in the end.

[Lucky Luke]

Originally posted by mmd:
Consult the designer before you go messin' with the rig - any amount he charges you will be worth it in the end.THIS is the one and proper answer ;

[Andrew S/Y Rocquette]

You also affect stability as you're making the centre of effort for the sailplan rise vertically. This will make her less stiff (i.e. you may need to reef earlier). Additionally, you're making it less likely she will balance on her own and may become more difficult to steer as low spread sailplans are more balanced around the boats centre of lateral resistance. So: in essence what the others said - consult a designer!

[Gerald]

I have consulted the designer. The kid that has taken over from the retired father sez "don't change anything." He has gone with the easy answer. I won't go into the designs of the kid because my objective is not to bash the designer but to glean some understanding of what I might expect if I chose to alter the design in some way.
I have already changed the design by moving the 370 kilo motor forward to the center over the keel. The 1500 litro fuel and water tanks that were above water line have been moved to below water line.
I have an 18 meter standing rig from a performance boat and will be cutting it down to fit this 37' boat.
When the boat goes in the water we will be heading south ................ way south ....... where there is ice and fair sized seas.
Gerald

[ahp]

I agree with the other posters on this thread. If you want to explore this question further look a copy of C. A. Marchaj, "Sailing Thoery and Practice", Dodd, Mead and Co, NY, 1964. In this book there are polar diagrams for sails of various aspect ratios.

[mmd]

Two questions, if I may, Gerald:

When you decided to move major weights around in the hull, did you calculate the effect this would have on the stability and roll period of the hull? Too much mass too low may make the boat's motion in a seaway uncomfortable, and the southern oceans certainly have dynamic seaways.

Is this the same boat in which you installed a Volkswagen auto engine that we had a discussion about a year or two ago?

Good luck with your project, I hope it all works out well for you.

[Dan McCosh]

Aerodynamically, raising the mast height and shortening the boom raises the aspect ratio and lengthens the luff. Again theoretically, this improves the lift/drag ratio as it reduces the tip vortex. This gain is offset by the above mentioned problems with moving the weight of the rig aloft and altering the center of effort. Whether it is an improvement depends on how the several factors offset each other--the essential compromise that the designer looks at.

[Gerald]

>>>>>When you decided to move major weights around in the hull, did you calculate the effect this would have on the stability and roll period of the hull? Too much mass too low may make the boat's motion in a seaway uncomfortable, and the southern oceans certainly have dynamic seaways.<<<<<
This seems to be leading to a similar discussion to the one we had about the motor. I ask about the prop and we got a two week forum discussion about motor size. However, it was worth while and anyone with an open mind learned something. To answer your question ..... No, I did not calculate roll period. I did calculate where items would be placed for the boat to float on the same water line. I also know that the center of gravity has been lowered, but I couldn't say exactly how much.

>>>>>Is this the same boat in which you installed a Volkswagen auto engine that we had a discussion about a year or two ago?<<<<<
No, the boat I installed the VW 1.9 in was a 32' 1967 Manual Campos Yawl. I have since sold the yawl without the motor. The motor performed great. It was smooth as silk. I only put 100 hours on it so who knows what might have happened with more hours? It did keep me out of trouble and got me to a safe port on the day of Brazils only recorded hurricane. The local Yanmar deal told the new owner that the VW was much to large for the boat so they have installed an 18 HP Yanmar. Heavier than the VW and he has already been delayed once because he couldn't motor under the bridge when the tide went out.
Gerald