Propeller question

[isha]

I have an Orkney Day Angler 21 powered by a Mariner efi 4 stroke 80hp outboard engine.
The Propeller is a Stainless steel one and stamped on it is 48-16988 18PW
At 4000 rpm it pushes the boat at 10-12 knots. Talking with others with similar sized boats I get the impression that at 4000 rpm I should expect something closer to 15 knots. I am wondering if I have the wrong prop for this engine and boat and if a larger pitch would be better. I have a prop with a smaller pitch and tried it out but it was even slower and would welcome any advice to help me decide do I leave well alone or should I try a different prop for a bit more speed.
Dave

 

[QBhoy]

Hi Dave
From instinct alone, I'd say there is something far wrong here, especially if you mean the boat is flat out or wot at 4000 rpm and about 15 knots.
I know enough to know that Orkney boats, although great sea keeping vessels, are not speed machines but I'd think that with that set up she should be pushing well into the 20's and maybe more.
At first read, I would have said that your prop has too high a pitch, but you also say you have tried a lesser pitch and it's worse.
Perhaps more details and figures are needed and maybe even a compression test on the outboard.

 

[Bouba]

The Mariner should have a wide open throttle of 5500 to 6000, if it reaches this then the prop is perfect, although the fact it's stainless means it's probably after market. Close to 6000 is best but it's over propped if higher. The sweetspot may not be at 4000 for your boat/engine/prop but your actual sweetspot may make your boat behave better than some one else's boat. For a small outboard just get the wide open in range. Only river people who never reach high revs need to think of getting bigger props i.e. Bigfoot

 

[MapisM]

Close to 6000 is best but it's over propped if higher.

You mean under propped, I reckon.
My understanding of under/over is when the pitch is too fine/coarse respectively.

 

[Bouba]

You mean under propped, I reckon.
My understanding of under/over is when the pitch is too fine/coarse respectively.

Yeah you got me, early morning brain

 

[isha]

The Mariner should have a wide open throttle of 5500 to 6000, if it reaches this then the prop is perfect, although the fact it's stainless means it's probably after market. Close to 6000 is best but it's over propped if higher. The sweetspot may not be at 4000 for your boat/engine/prop but your actual sweetspot may make your boat behave better than some one else's boat. For a small outboard just get the wide open in range. Only river people who never reach high revs need to think of getting bigger props i.e. Bigfoot

The engine is a 2006 model and I think has probably been used on a licenced fishing boat. The prop is a Mercury one. I had to fit a new water pump assembly as the old was well salted up. Since then (2 years ago) it has performed well but I never push it above 4000 rpm as am not sure what rpm I can go to since its an older engine with no service history and my first 4 stroke. (Previously had a 2 stroke 75 hp Yamaha on a Predator 165 - went like stink). The boat is out for the winter and though I am tempted to seek a younger engine in the hope of a better performance it's only in calm weather I would push for a faster speed as in rough seas, and they are often that, I am only using about 3000rpm and 10 knots is probably fast enough for an old guy like me.
Thank you for your helpful reply.

 

[QBhoy]

Ah, got you now. That's a different story
2006 isn't generally considered old for an outboard. Spring chicken, 'more like. Although you did say it has been used commercially.
Best thing to do is open her up come the new season and jot down figures. You will soon see what is right or wrong and what suits you best.
Just a slight thought...at those speeds you may be working the engine harder than it would be when it's at higher speeds and up and over the hump on the plane.
All the best.

 

[isha]

I agree that I should grit the teeth and open her up full throttle and see if she does the revs as Bouba says 5500 to 6000 rpm and take it from there. Thank you

 

[Bouba]

I once had a brand new Honda 150 that the boat yard put the wrong prop on and I couldn't go over 4200 rpm. The right prop put her back to 6000 and she was a sweet engine until stolen three months later. I wish I could find all my prop notes, I would go round the port checking on everyone's prop till I had the right one. When I got my new motor it had the correct prop made by Honda, it was perfect. I would go to a mariner/mercury dealer and get a prop from them, it doesn't need to be expensive stainless, often the dealer has old props for you to try.

 

[isha]

I once had a brand new Honda 150 that the boat yard put the wrong prop on and I couldn't go over 4200 rpm. The right prop put her back to 6000 and she was a sweet engine until stolen three months later. I wish I could find all my prop notes, I would go round the port checking on everyone's prop till I had the right one. When I got my new motor it had the correct prop made by Honda, it was perfect. I would go to a mariner/mercury dealer and get a prop from them, it doesn't need to be expensive stainless, often the dealer has old props for you to try.

It would be great to be able to do that but there are no Mariner/Mercury dealers where I am. Yamaha, Honda and Susuki.
Perhaps I could contact a Mariner dealer in the hope that I find someone who can advise and I am happy to purchase a prop on good sound advice.

 

[gordmac]

You could try asking here: http://www.worldseafishing.com/forums/forums/boat-owning-equipment-and-maintenance.77/
I would say until you know what revs it pulls at wot you won't know how suited the prop is. I would also suggest if it is a planing hull avoid about 6-15 knots, run at hull speed or planing not in between.