ventilation problem

[superchas]

I have a 25hp s/shaft yamaha[with a larger anti-cavitation plate installed] on a 3.6m inflatable [inflatable keel] with a 15" transom .this engine unfortunately ventilates @ 1/4 throttle.The prop I have is alum 9 7/8 x 10 ! I also have a 9.9 x 12 prop but neither work without ventilation.I need a prop that can take a fairly heavy load [ie up 2 divers + 4 tanks] as I am nearly bald from pulling my hair out trying to find a solution I seek some help from all the "techowizkids&grandadsetc" out there.

 

[steelespike]

Re: ventilation problem

Have you tried it with the extra antivent fin removed?
Is the actual height from top of transom to the water when planing about 15"
If it is venting at 1/4 throttle something is wrong with the setup.Some part of the boat or motor disturbing the water.Or the boat requires a 20" motor.
A prop with some cup will combat venting.Without actual rpm and speed at wot we can't suggest a prop pitch but I would start with a 10" pitch.From your description its not likely to solve the problem.Hustler props combat venting.
You might check that you don't have a spun hub indelable marker across the underside of hub run it and see if the line still lines up.Not likely both props are spun but take a look.

 

[superchas]

Re: ventilation problem

Well I did have a 30hp L/shaft engine on it and had none of those problems [the previous owner installed] but then started to have overheating problem[another story] any way I purchased a 25hp s/shaft as I thought would be more suitable as the long shaft engine is very heavy.The "permatrim" A.C.P is not removable [ie bonded].I could transferr L/S to smaller 25 as a last resort or lower transom but I wish to seek an alternative.I am not sure I undrestand your referrence to hub etc but I will check.Does anyone sell Hustler in NZ? [I presume thats not purchased on the street corner]

 

[steelespike]

Re: ventilation problem

Most props have a rubber hub that slips to protect the drive train if something hard is struck.Some times the start to slip.If you draw a line across the hub if the hub slips the line will no longer line up.
I think we have your problem solved.Your boat appears to require a long shaft.What does the transom measure from top to the actual bottom of the boat?There are other props, those with cupping that resist ventilation.
I mention the Hustler because people have good luck and they are reasonable.They are sold here at iboats,don't know about shipping.
From your description I don't think a prop will solve your problem.
If you put a straight edge along the bottom toward the prop I think you will find the upper blades would be out of water as they rotate.The long shaft is 5 inches longer a prop can't work if its not in the water.

 

[superchas]

Re: ventilation problem

Thank you for your help! I think you may be right that this is not a prop problem!The transom measures 15" the A.C.P sits 2" below bottom of transom however keel [inflatable] may sit below that,you cant tell when it sitting on trailer[sits flat].I thought a full 2'' was enough clearance below but looking at it travelling in water the A.C.P does break the surface & only operates on bottom rungs of the trim the best being the very bottom .The curious thing is other surf rescue boats [like mine] employ a short shaft engine with a stainless prop [may be shorter transom?] & they work fine in the adverse conditions they operate in.I will test the prop with a line but one of my props is brand new so I doubt that it is slipping so I guess it is back to a 20" shaft unless you have any other suggestions ?[other than lowering transom as this is fibreglass]

 

[steelespike]

Re: ventilation problem

The allmost imeadiate venting problem tells me the sertup is wrong.
Probably shaft length. Is there a height adjustment on the motor itself?
The inflateables do sometime disturb the water enough to cause problems.
A jack plate might help your problem.It would put the motor in cleaner water and allow further adjustment.Wheather tiller operation would work I don't know.

 

[superchas]

Re: ventilation problem

by a jack plate I presume you mean like an aux bracket! Well I have found one that Tenob make that would extend & lower engine in water rated to 25hp usually used on yachts or bigger vessels.I would need a extension arm on the tiller as well.It starts getting expensive but I now looking @ lowering transom as well .I dont think that there is any adjustment on yam 25hp down.I will look into that.Ofcourse I could also put longer shaft [as I have it ] on 25hp but separating it[from the 30] is proving to be quite a job as its well and truley married.

 

[steelespike]

Re: ventilation problem

by a jack plate I presume you mean like an aux bracket! Well I have found one that Tenob make that would extend & lower engine in water rated to 25hp usually used on yachts or bigger vessels.I would need a extension arm on the tiller as well.It starts getting expensive but I now looking @ lowering transom as well .I dont think that there is any adjustment on yam 25hp down.I will look into that.Ofcourse I could also put longer shaft [as I have it ] on 25hp but separating it[from the 30] is proving to be quite a job as its well and truley married.

No, I was refering to a jack plate.Designed to mount the motor farther back and it allows easy adjustment of the height.You may only need to lower an inch or 2 the main question is will the set back make using the tiller difficult.

 

[steelespike]

Re: ventilation problem

by a jack plate I presume you mean like an aux bracket! Well I have found one that Tenob make that would extend & lower engine in water rated to 25hp usually used on yachts or bigger vessels.I would need a extension arm on the tiller as well.It starts getting expensive but I now looking @ lowering transom as well .I dont think that there is any adjustment on yam 25hp down.I will look into that.Ofcourse I could also put longer shaft [as I have it ] on 25hp but separating it[from the 30] is proving to be quite a job as its well and truley married.

No, I was refering to a jack plate.Designed to mount the motor farther back and it allows easy adjustment of the height.You may only need to lower an inch or 2 the main question is will the set back make using the tiller difficult.
Its possible the set back may be enough to solve the problem.

 

[superchas]

Re: ventilation problem

Well at last success.I took the bold step and I took to the transom with a jigsaw & and carved out a 1.5" knotch.To my surprise I found that the transom is actually glass over ply which is good as I thought cutting glass can get very messy and produce cracks!I then did a sea test & found with the boat on a light load [ie just me ] on the bottom rung I got full throttle for the first time.I think fully loaded it should perform better.Thank you for your help Admiral Steelespike in steering me in the right direction .Now to get back to some serious diving & fishing on the hauraki gulf [auckland nz].I presume you know of it because thats where we wipped you boys on the water & stole the americas cup any way have a happy new year!

 

[steelespike]

Re: ventilation problem

All ways great to see a success.Just Soak the ply with resin and put a layer of fiberglass over the cut. If the ply gets wet it won't take long to rot.