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Old 09 April 2014, 17:14   #21
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Originally Posted by captnjack View Post
It still seems a little high but ok.



You are bow steering. On a normal SIB this would happen with too much weight in the bow or the engine trimmed down too much. In your case its because of the speed tubes and the fact that you want to travel slow under bridges. You altered the boat so much it doesn't want to go slow. Its meant strictly for racing now with the bow off the water and only the rear portions of those speed tubes in contact with the water as you fly along at full throttle. If you want those (very large for the size and weight of the SIB) speed tubes then you are going to have to drive it much much faster to create the bow lift and give you directional stability. If you want to go slow and stable then cut off the speed tubes. You can't optimize the hull for both at the same time.
This makes alot of sense to me
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Old 11 April 2014, 09:40   #22
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Motor too low

Your motor is way too low. I have a round nose twin hull aurora ( similar to Gemini) and a bottom starting point is Center of prop to Center of hijacker. Then slowly lift motor to your liking.
My boat used to wander and fidget a bit and it's because the fat part of the leg was getting water hitting it.you should end up with the best all round boat. The round nose are great for keeping spray down, efficient at speed, more stable and far softer riding especially in chop.
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Old 11 April 2014, 09:57   #23
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Originally Posted by Bos suzi View Post
Hi guys.

I am new to this forum and would really like some help from experienced inflatable boaters.

I have 3.3 m fully infalable with a 15 hp 2 stroke,

I recently installed small pontoons underneath (high jackers ) to create a tunnel hull, this was done professionally by one of the companies that specialize in tunnel hulls for racing.

I currently am strugling with handling issues,

It is realy difficult to to steer and have directional stability. If i am on the dam , it is realy hard to drive underneath a bridge as the boat flanks left and right and is extremely sensitive to any movement on the throttle in terms of steering.

My highjackers was around 0.8 bar, should it be pumped harder?

Should i avoid sitting on the oversized pontoons?

Should i try lowering the engine deeper in the water?

I am really worried , and would love just to have directional stability,

Can this be caused by running a trim which is too negative !?

It feels as if when i am on the plane, that the tunnel hull suddenly bites on the water trying to then change to direction of the boat.

Please help ,

Thank you.
Its funny i was thinking only the other day about purchasing some hi-jackers to add onto one of our SIBS and i have now gone off the idea. Couldnt find anywhere in the UK to buy them anyway..
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Old 11 April 2014, 10:14   #24
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I'd say the motor wAs too low. The fin above the cavitation plate is just below the level of the deck. The hijackers on my thundercat are 0.8 bar as well. Try trimming the engine up a little as it'll lessen the steering.

I've never had the problem you've had though with my thundercat.

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Old 11 April 2014, 17:37   #25
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Originally Posted by lukewhiting View Post
I'd say the motor wAs too low. The fin above the cavitation plate is just below the level of the deck. The hijackers on my thundercat are 0.8 bar as well. Try trimming the engine up a little as it'll lessen the steering.

I've never had the problem you've had though with my thundercat.

Attachment 93226Attachment 93227
Thank you, ill try trimming the engine upwards, I need to go to the dam , and see if my motor still gets enough water, coz the water intake is on the other side of the cavitation plate. I think the flumzy front deck at the bow also isnt very helpfull.

Tell me i assume with a tunnel hull, sitting on the oversized pontoons should be avoided and i should sit on the floorboards?

Thank you,
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Old 11 April 2014, 17:59   #26
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Originally Posted by Bos suzi View Post
Thank you, ill try trimming the engine upwards, I need to go to the dam , and see if my motor still gets enough water, coz the water intake is on the other side of the cavitation plate. I think the flumzy front deck at the bow also isnt very helpfull.



Tell me i assume with a tunnel hull, sitting on the oversized pontoons should be avoided and i should sit on the floorboards?



Thank you,


Is the deck an air deck or solid? Give sitting on the floorboards a go. I do & you get better steering control & the boats more level
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Old 12 April 2014, 09:12   #27
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Quote:
Originally Posted by Pedrotor View Post
Your motor is way too low. I have a round nose twin hull aurora ( similar to Gemini) and a bottom starting point is Center of prop to Center of hijacker. Then slowly lift motor to your liking.
My boat used to wander and fidget a bit and it's because the fat part of the leg was getting water hitting it.you should end up with the best all round boat. The round nose are great for keeping spray down, efficient at speed, more stable and far softer riding especially in chop.

Can you please explain the part about the fat part of the leg hitting the water?

Also how did you resolve this? With the correct trim?
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Old 12 April 2014, 11:50   #28
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A semi cleaver prop gives lots of bow lift and might work if you have other compelling reasons to try one - ie prop ventilating.
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Old 12 April 2014, 11:53   #29
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Nope sorry they give stern lift. Ignore me!
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Old 12 April 2014, 14:16   #30
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@ rik_elliott i cant find any semicleaver or cleaver prop for 15 hp, in other words, 10 inch or 9inch pitch
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