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Old 22 April 2007, 19:13   #11
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As mentioned before, Smart tabs make a huge difference, easy to install and relatively cheap. They also allow you to plane at lower speeds.
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Old 22 April 2007, 20:33   #12
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Smart Tabs look like a good choice , will look to make sure cavitation plate is at level (or below) of V of stern. How do the Smart Tabs effect towing? My next task is getting a clear understanding of whether I can safely tow the boat behind a 40' powerboat at 24 knots? I've been a sailor for over 15 years and power-boating is been only the last 5-6 years. I will see if I can get some ideas here before posting a new thread. My first step is figuring out how to reinforce the bow to withstand the forces of the towing.

My options are for local towing do nothing since I can get to all of the USVI and the BVI without that much boat speed. However, when I travel to the Leeward Islands I really need to cruise at +/- 24 knots to make the 85 nm passage in a reasonable amount of time. My wife and I aren't trying to buy an 8' RIB with a 9.9 - 15hp just for this, seems like a waste. Not to mention storing it when its not needed.

--Chris, no more vacations in the VI, moving there!
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Old 31 January 2008, 12:18   #13
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If the anti cavitation plate is too high, the boat will cavitate even more if you get on the plane. I think the problem is that all the weight is near de transom. If I were you I would take the boat out on the flats and play a little with it.

I had a 40hp Yamaha 2 stroke behind my Brig F360 (no console) which is only 175 lbs (80 kg) and with the right prop it hits 44 mph on the GPS, and the bow rises extremely high while getting on the plane. Took me a while before I could handle it, it's best to trim your motor in, to get the nose down while getting on the plane our cruising at high speeds. That's the way I learned to drive the boat. The padded v-bottem is a little tricky for beginners at high speeds!

You could also ask some people too cruise along, that will make your boat easier too handle. @3800rpm the boat won't go anywhere near speeds that will make a Brig F450 dangerous to drive!
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Old 31 January 2008, 14:58   #14
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Firstly it sounds like your engine is trimmed out if the boat if the bow is lifting up as you say so it is... make sure you are trimmed fully in when trying to get on the plane and that when trimed fully in the leg of the engine is parallel to the transom.

Secondly I am really confused that you are reaching "frightening" speeds at 3800 RPM as at that RPM I would be suprised if you are pulling more than 25knots. Again this makes me wonder if your leg is trimmed to far out and as a result the boat feels very loose on the water which would make the boat feel faster due to it being unstable.

Can you take a picture of the engine from the side when it is fully trimmed in and post it on here?

Also look at the height of the plate above the prop and compare it to the bottom of the transom it should be level with the bottom of the transom or maybe an inch or two above the bottom of the transom.

I would not go changing props yet and certainly dont bother fitting trim tabs until you sort out the rigging of the engine which is clearly not right.
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Old 31 January 2008, 18:04   #15
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Quote:
Originally Posted by dawktah View Post
All info says go to a 19 or 21 degree prop, since that will lower my rpm at WOT.
Don't go fitting tabs or foils until you've got a reasonably running boat.

Do the checks on the engine trim as suggested. Be aware that most outboard motors have a pin through the lower part of the mounting bracket which limits the amount of 'in' trim which can be applied. If this pin is too far outward, even full 'in' trim can still be too far 'out'. I hope I've explained that ok. The pin can be moved to an alternative set of holes to make adjustment. The engine can satisfactorily be mounted with the plate below the bottom of the V if necessary, although folk tend to go for high positions generally, 25mm below the V is still a good position in order to bury the prop more.

However, if all engine rigging is fine and you are happy with your boat driving ability, the characteristics you are describing would lead me to reduce the propeller pitch. What you are experiencing sounds like the prop breaking away because it is too highly loaded and this causes ventilation (not cavitation). When/if the boat gets over the hump and planing, the load on the blades is lowered and it may regain its grip on the water. It may also account for you feeling the boat is very quick for 3800rpm. It could be you're closer to WOT than you realise.

The cavitation plate is properly known as an anti-ventilation plate. Cavitation is an entirely separate phenomenon.

To sum up, make sure the 'in' trim is correct, engine height is correct and then, if necessary, try a lower pitch prop.
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Old 31 January 2008, 19:06   #16
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And nine months later, out popped another post
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Old 01 February 2008, 04:33   #17
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Doh! just realised.
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Old 01 February 2008, 05:08   #18
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Me too...

That's what happens when folk don't have the courtesy to finish a post they've started.
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Old 01 February 2008, 10:05   #19
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Doesn't matter, it was interesting reading for me. Thank you all.
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Old 01 February 2008, 18:16   #20
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so many post like this never have an outcome , everyone does thier best to help but we never know what the real problem was
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