# diy surface drive.

Discussion in 'Surface Drives' started by mike08kurtz, Dec 22, 2014.

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### TANSLSenior Member

I expected no less from you. You have fulfilled all my expectations. Now I know what is that. And now, please tell me, what does that have to do with what I have outlined in your sentence ?. Whatever it is a balance, it is difficult for the boat hanging up horizontally. I have to repeat that: "what you wrote, and I have underlined, is not correct".

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### gonzoSenior Member

It's the kind of experiment most kids do in elementary school. The scale is horizontal when the pivot point is vertical to the CG. I know you don't usually understand these things.

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Location: Anamosa Iowa and North Buena Vista on the Mississi

### Dave TSenior Member

A simple way to find the center of gravity for the boat being it looks like it is on a two wheel trailer is have the trailer on a level surface without the boat on it. Put a scale under the trailer jack a bathroom scale will work, put a level on the trailer frame and adjust the jack to level the trailer and record the scale reading. Then load the boat but leave it back so it is lifting the front of the trailer slightly, put a jack under the back of the trailer and unhook it lower the rear jack to clear the hitch and pull the vehicle out of the way. Place the scale back under the tongue jack and winch the boat ahead until it just puts a little weight on the front of the trailer level the trailer and then slowly winch the boat ahead until the weight is the same as the tongue weight when the trailer was empty at this point the center of the trailer axle will be the center of gravity of the boat. You could use a plum bob to mark the center of gravity on the boat. The more accurate the scale the better and you could then pull the boat ahead an inch to see how much the scale changes to give you a margin of error.

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### gonzoSenior Member

Interesting method. You could set the trailer to have only a couple of pounds of tongue weight when empty. Then use a kitchen scale which are much more accurate.

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### Brent AZJunior Member

Gonzo- Thanks for your suggestion. User TANSL appears to be joking with you. If my boat were completed with the engine, drive, and interior all in place, then I could try balancing the boat about its CG. But the boat is not at that stage yet. I do have a good estimate of the weight for most of the component's weight that will be mounted in the boat and their longitudinal distance from the transom. If the hull was bare, I could fill some sand bags or buckets with sand weight to match the individual component's weight and place them in the approximate location to simulate the finished boat setup, then I could do the balance beam method to find the CG.

A variation of this balance beam idea which would be easier and safer to perform is to use a round pipe under the keel of the boat. The boat can be rolled forward and backwards on the pipe to find the balance point as long as you have helpers to steady the boat to keep it in the upright position as it will want to rock side-to-side while balancing on the pipe. I'm thinking this method might be easier to steady because the boat is not suspended in the air. Suspending the boat in the air would require extra men to also steady the boat from rocking side-to-side as well as front-to-back. The boat would have to be lowered to the ground, supported at each end to allow clearance so as not to bind the sling between the boat's keel and the ground, in order to re-adjust the sling forward or backward trying to get it on the boat's CG point.

I was hoping for a different method that would allow me to perform some pre-build design analysis. Any other ideas?

My current plans to have the engine, transmission, casale V-drive, windshield and driver's cockpit, gas tanks, batteries, almost everything except for the front wrap-around bow seating will be in the rear half of the boat. This means that the boat will be rear heavy anyway. So I am not expecting that the boat will have any problems carrying the bow even without a cavitation plate or trim tabs, but load-up the front seats with passengers, then how much trim tab surface will I need to balance that load?

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### gonzoSenior Member

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### WestVanHanNot a Senior Member

A friend wanted to do this exact thing a few years ago,I talked him out of using a pipe as the entire weight of the boat is resting on about 1/4" of the pipe. Something may be damaged.

I suggested a 2x8 or 2x12 to at least spread the load out. A couple guys could lift the bow with another to slide it back to find the centre point, to within an inch or so.
Close enough.

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### Brent AZJunior Member

WestVanHan your point is well taken about the entire boat's weight being applied to a small point on the keel might cause damage. My boat's empty hull only weighs about 1200 pounds, but even that amount of weight might leave depressions when resting on a very small spot. So I will use lifting straps to hold the hull suspended to find the CG point. I have an overhead crane that will hold 3 tons that can be rolled in place with the boat below it. The friction of the straps will probably do a good job of keeping the boat from rolling side-to-side while the boat is suspended in the air. I can put a support at the front and back of the boat to rest the hull upon so that there will be clearance under the boat to allow the straps to be slid forward or backward using several trials lifting the hull in the air to see if it balances, lowering it back down on the supports and re-adjusting the strap location with each trial until the static CG is found.

I plan to add sandbags with the appropriate weight of sand placed on location in the hull to simulate the weight of the engine/drive components and interior seating, fuel load, passengers, etc. Once I have found the boat's static CG I will have the information to allow me to do any calculations for moment arm forces relative to the propulsion and trim tab forces.

Does anyone have any suggestions for me regarding the size of trim tabs and how much force the tab size will apply to the hull? I know that the tab size, forward speed, and angle of attach of the tab will develop a specific force applied to the hull. Also the propeller location and developed pushing force will apply propulsion forces. Assuming that I use an Arneson ASD6 surface drive with a 16.5" diameter X 26 to 30 inch pitch prop. Also assuming that my hallett boat has a gross weight of 3500 pounds. Knowing the force produced and the distance from the CG will result in a specific hull attitude. Having been a product development engineer I know enough to understand the design principles to be dangerous, but not having direct experience in boat design leaves me lacking in the specifics of trim tab sizing, therefore I need some guidance from those who have this experience.

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