Shaft length for inflateable boat

[Toolboy]

I have a 14' long zodiak style inflateable boat. It is rated for a 30HP motor. I have been given a 5 year old Tohatsu 25HP motor with a long shaft.<br />My question is, what problems (handling) will happen when I use the long shaft?<br />Would it be worth it to have the shaft changed?<br />

 

[phatmanmike]

Re: Shaft length for inflateable boat

well first off let me welcome you, i also have a inflatable witha nissan outboard.. !! <br /><br />if your shaft is too long, you will not have as high a top speed because of drag and you wont be able to go in as shallow of water<br /><br />you could possibly build up an extension on the transom, although i dopnt know how it would hold the weight of the motor, or sell/ trade that for another one of proper shaft<br /><br />somebdoey else will know more than me<br /><br />always do

 

[Jdeagro]

Re: Shaft length for inflateable boat

The bow rise wil be very high at pre-planing speeds. Getting the boat to plane will be an issue and staying on plane at less than 17 MPH will be a problem. <br /><br />If you stay with the longer shaft motor then you will need trim Tabs (Smart Tabs). Hydrofoils will not do too much.

 

[SPYKE-KPT]

Re: Shaft length for inflateable boat

Nautijohn, Question, you say hydrofoil will not do you any good can you tell me why. The reason I am asking is I just bought a 1984 16' achillies which is rated for a 60 hp. I found a 50 hp. and had it put on. It had a hydrofoil on it. I took it out for the first time last week and was going about half throttle and it started to cavatate and water was comming over the transom so I then started to play with the trim to get it to stop.<br />When I did half of the transom came unglued almost lost the engine. Everone I talked to said that thy nerver heard of an achillies comming apart like that so I think that the hydrofoil has somthing to do with it. What are your thoughts?

 

[Jdeagro]

Re: Shaft length for inflateable boat

Spyke;<br /><br />First of all i do not think that the transom delamination was the cause of the Hydrofoil. Just get a good Hypalon Glue and clean everything really well, then re-glue, or take it to an inflatable repair facility. Call the manufacturer for a list of repair places.<br /><br />Next; when the motor shaft is too long the prop has excessive leverage. The shaft and the prop are pushing from a position well below the bottom of the boat and trying to lift the bow. The best place for the prop is as close to the center of the boat as possible. Up - down and side to side.<br />If you draw it on paper you will see what I mean.<br /><br />The hydrofoil will not provide enough stern lift to truly affect the bow rise. It is simply to small in surface area. <br /><br />Infaltables and Ribs are light and the weight balance is very far to the stern. They also have a small foot print in the water for the amount of load they are expecte to carry. Therefore it is harder for them to plane and stay on plane (PSI is high). The more surface are you can add to the hull (ie: trim tabs) the easier it is to plane, especially if that surface are can be adjusted to go up and down as needed (like plaps on an airplane wing. The deployed angle of the trim tabs during acceleration provides aggressive lift at slow speeds, then when on plane they come up to horizontal and lift without adding drag.<br /><br />Smart Tabs are automatic and now used by a number of Rib manufacturers, law enforcement, and the military because they make these boats more efficient. <br /><br />One last thing regarding hydrofoils. The technowlogy is not new and is usefull, but needs to be integrated int the design of the boat. Not to mention that you never want to lift the boat from the motor (center of the boat). The boat should be lifted from the hull.<br /><br />If you would like a copy of a tech report on boat ballance and performance send your address to John@nauticusinc.com. There is no charge for the report. To is free to anyone!

 

[Link]

Re: Shaft length for inflateable boat

Welcome to IBOATS Toolboy<br /><br />Handeling problems have been addressed using a long shaft vrs short shaft. Except that with a light inflateable times it by two at least.<br />Your work on the tiller!<br /><br />Now lets get a quick fix for your long shaft problem.<br /><br />The cost will be from about 50 to 200 dollars.<br /><br />I don't know your boat but if the transom that you hang your motor on ISN'T part of keeping your boat dry inside and water tight. And is just bolted to the outside pontoons. Then just buy another peice of the same material and cut it out 5 inches higher, problem solved. Use the same bolts.<br /><br />Other wise you have two quick options that will cost about 200 bucks. <br />A: Go to a local aluminum boat builder and have them build a transom plate (which I did) and bolt it to your existing transom and hang the motor from it. about 200 bucks<br />B: Buy a transom jack plate (CMC makes a good one) and bolt it on. another 200 bucks but raises the motor 5 inches.<br />Toolboy:<br />I respect the heck out of these people on iboats but sometimes a simple fix is best. And if there are holes in my fix they will correct me <br />Which is why I try to read every night. <br /><br />Link

 

[frigatecdr]

Re: Shaft length for inflateable boat

I posted a question about 18hp vs. 25hp for inflatable. Tried your 25 yet?

 

[Toolboy]

Re: Shaft length for inflateable boat

I have not tried my 25hp yet. I am waiting for the electric start to be installed. Not to mention I have not even recieved the boat yet. The boat is a Titan inflatable.

 

[Solittle]

Re: Shaft length for inflateable boat

Well - I run a 13' alumnium runabout with a 1975 9.9 Johnny long shaft. It violates all the "should be" rules but runs just fine (on plane) with the wife & I aboard thank you very much.