SKI BOAT PROP - HELP

[MESS147415]

hello guys - ive just got my ski boat on the water for the first time after a winter restoration project. engine is an inboard marinised 3.5 litre V8 rated at 150bhp direct drive thro hull bottom to 3 blade bronze prop which measures about 13 inch dia. boat is very quick to plane and will do about 35mph at 5000rpm (not full throttle).<br />Two questions, sometimes when accelerating, particularly when or just after turning, there is a hevy rumbling from the prop area which goes if i ease back the revs - is this cavitation and what can i do about it?<br />secondly, is my prop too small? as a guideline what size prop will give me best all round performance for pulling skiers/boards etc<br />thanks steve

 

[sangerwaker]

Re: SKI BOAT PROP - HELP

Prop could be slightly bent or out of balance. Inboards are very fussy on props. Some props right out of the box sound "growly".<br /><br />What is the make and model of your boat? I can point you in the right direction with that info.<br /><br />13 inches is roughly what most inboard ski boats use for a prop. You'll need to know the rotation and shaft diameter also. Some manufacturers spin the prop clockwise (Ski Nautique), and some spin counter-clockwise (Sanger, Malibu). Most prop shafts are tapered and either 1" or 1-1/8".<br /><br />I would highly recommend an Acme prop if you decide to try a new one. They perform excellent and are quieter than most because they are CNC machined. If it's possible, go to a ski boat dealer and see if they will let you try out some different props before you purchase.<br /><br />The best prop for your boat will be the diameter and pitch the manufacturer recommends. From there, it's a question of material (nibral or stainless), 3 or 4 blades, and brand.

 

[MESS147415]

Re: SKI BOAT PROP - HELP

hi Sangerwaker - boat is an old Delta Clubman - 16 foot. they are no longer in existence so no help from that direction!. rotation is counter clock and shaft is 1". I have had it dynamically balanced and visually it is ok. I have also fitted a new sterntube bush and re aligned the engine/gearbox to the shaft coupling - it is a rigid coupling as the gearbox carries the thrust load. Performance is acceptable apart from when this rumbling starts after aggressive turns! steve

 

[sangerwaker]

Re: SKI BOAT PROP - HELP

Sounds like you know your way around your boat...excellent.<br /><br />Could very likely be your prop ventilating or grabbing air. You could try having a prop shop add a small amount of cup to your prop.<br /><br />How long have you had the boat? Has it always done this or is this something new?

 

[MESS147415]

Re: SKI BOAT PROP - HELP

Its the first time ive had the boat on the water so dont know if its always done this! What is adding cup please? thanks steve

 

[sangerwaker]

Re: SKI BOAT PROP - HELP

Cupping is adding "bite" to the trailing edge of the prop. The trailing edge is bent upwards slightly to help acceleration, ventilation, and usually efficiency.<br /><br />Sounds like your problem is normal for that boat, and a lot of other inboards. I wouldn't worry about it.<br /><br />You could always buy another prop (it's nice to have a spare anyway) and experiment with it.

 

[MESS147415]

Re: SKI BOAT PROP - HELP

Thanks for your help sangerwaker - I'll live with it and just enjoy the noise the V8 makes! After last year with a 70Hp outboard this boat is the business - Can pull both kids with ease and still stretch their arms a bit! steve

 

[sangerwaker]

Re: SKI BOAT PROP - HELP

No problem Steve. Get those kids out there and enjoy the boat!