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Revs vs Speed vs Prop

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Orkney Strikeliner 16 

30HP Yam 4S outboard should rev 5000-5500 wot

Hull not deformed 

Outboard raised up so plate online with bottom of hull

Outboard serviced, not balanced carbs but otherwise running well no leaks.

Set up:
Yamaha 30hp four stroke outboard
25L fuel & battery stored to stern 

25L fuel stored bow locker
Leisure battery in bow locker
10m 8mm anchor chain with 7.5kg Bruce anchor in bow storage with 40m rope

Clean enough hull, copper coated
Outboard lined up with keel

Prop Test:Run both ways on an estuary, results same each way. Revs taken from digital Yam rev counter, speed taken from Dragonfly 7 Pro


Prop results:

Prop 1 Genuine Yamaha
Prop Size: 11 3/8 x 12” 
Revs: 4400 
Knots: 16-17
Load: 1 Man (13st)

Prop 2 Genuine Yamaha
Prop Size: 12 1/4 x 9" 
Revs: 5000 
Knots: 15
Load: 1 Man (13st)

Prop 3 Michigan Marine
Prop sizes 10 3/8ths x 11” Three Blade prop with Yamaha hub kit adapter. 
Revs: 4500
Knots: 14-15
Load: 1 Man (13st)

Prop 4 Genuine Yamaha
Prop Size: 10 5/8 x 12” 
Revs: 4400 
Knots: 11.2
Load: 1 Man (13st)

No water in boat

Have removed 30kg of ‘stuff’ from bow locker not gone out since then.


Local mech says 5000 rpm best I’m going to get

Michigan Marine says something wrong with outboard

Other Strikeliner owners getting 18-22knots with similar set up

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When I had an Orkney 440 with a 20hp Honda on the back I don't think I ever got more than about 12kts out of it. Something in my brain tells me that Orkney's are just a slower hull design but I have no evidence back that up.

Try again without the 30kgs of stuff on board and see what happens.

Also try opening any bungs to check the hull is truly dry. I emptied a load of rainwater from my Orkney that had gathered in the triangular voids either side of the bilge/fuel tank cubbyhole. The under-deck area was dry, but when I removed the first of the bilge pump fixing screws to replace the diaphragm a stream of water shot out.


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Boat was out of water for quite a while and hull doesn’t have much inside even if full. Like 10L or so, un thought about weight can be cumulative in that way so there’s that to consider.

Yes those sponge cells I hear just soak up water, owners have posted deteriorated blobs to prove it. Probably not fit for purpose and might hold 5L or so per side? 

Have got 25L fuel, 110amh battery in the stern but moving fuel midships doesn’t change things, moving a crew member to the bows didn’t either 

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According to the spec on that engine, max rpm should be between 5000-6000. When I prop my boats I do so at the high end of that range, which none of those are doing. According every outboard mechanic Ive spoken with, propping them so the rpm is lower than that range is more harmful than higher ranges. I would be investigating your prop range more and try to find either a lower pitch or lower diameter to get the rpm up to 5500 at least. If there are no props in that range you could consider selling for something bigger. Even though your plate is level with the bottom of the boat, you may find raising one or two more holes will reduce drag and bring a few  rpm. If you go to high and it starts sucking air into the prop, you can easily drop it back down one.


A 30hp is a huge ask for a 16ft boat with the weights you mention. Under powering boats seem to be so common in the UK, I had it with every boat I ever bought there. It's not just the weight though as the more surface area, more drag. Different hulls obviously but the 90 honda on my 15ft was underpowered, hence going to a Yam f115.

Have you tried the Yamaha prop selector  ?  https://www.yamahapropselector.com/propSelector/web/inner.html

You seem to be using a larger diameter blade than options I came up with for that engine.


Edited by JDP
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Mentioned above that 30hp is a huge ask for a 16ft boat with the weights you mention.

The Strikeliner is a.semi displacement hull that was rated for 25hp.  Many owners put a 30hp Tohatsu 2 stroke on.  However your 30hp 4 stroke will be quite a lot heavier than the recommended 25hp 2 stroke.  In a favourable sea and very lightly loaded my 25 gave my Strikeliner a top speed of 17 knots.  It was a fantastic boat

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