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Prop choice

Hi, I have been looking at old posts and am hoping I can get some help here. I have a 2stroke 1980s 40hp mariner on the back of my 16 foot 6 Broom Aquarius speedboat. I tested her for the first time on the weekend. I am only getting 11knots out of her at full throttle. I feel this is a little slow and so plan a tune up. This I am happy with but prop choice I have no experience with. The prop supplied is pretty shabby so I am looking to change it. The only markings on it is PR505 a sixth digit/number is unreadable. My thoughts are that the prop may be no good for my set up. Any thoughts on what my prop is and what prop I should receive would be gratefully received.
 
Maybe a little under powered,they're a pretty heavy boat for 40HP,most I see for sale have 90HP+ hanging on the back.
I'm going to guess you're located in UK,check out these guys,http://www.cmba-uk.com/ ,a fair few Broom's owners on there
 
Thanks for your replies. I have a Johnson 85hp v4 that I am stripping and rebuilding. From the bottom up when I find the time. I have only serviced the gearbox so far, (3 kids and myself and my wife in full time employment). So do intend to fit that in the future the mariner will have to do for now. It's only for light family use, spot of fishing and going up river. I am in the uk Plymouth. I have tried cmba allready. I dont think my boat is enough of a classic on there. Any thoughts on the correct prop size and pitch? Thanks
 
Hah,if my auld Shakespeare is enough of a classic,your broom definitely is.A lot of them,like yourself,working to keep the powers that be well fed.
As Jeff says,lot less prop,though you're not going to get a great deal of speed.
What you need to make sure of is that your prop is putting you engine within the RPM's range for your outboard WOT.Which,again I'm guessing is a Yamaha built Mariner,and off the top of my head should be 4800-5500,I'm sure I'll be corrected if I'm wrong.It'll say "Made in Japan" and the number will start with the three digits 6E9-xxxxxx
 
You are correct it's an 1986 6e9 Yamaha built mariner. I am not fussed about top speeds. I am more concerned with the wrong prop damaging my engine. I think a 40 should get the boat up on the plane though.
 
I ran your numbers,again guessing the weight of the boat,through a propeller wizard,and it threw up 10.5"x8P. Now this is not exact science as there are so many variables and my guess of the weight of the Broom ,yourselves,what you put on board could be way out.So you can calculate yourself on here, http://turningpointpropellers.com/PROPWIZARD/ .It's not exact,but will give you a starting point
 
Brilliant thanks for that I tried to a couple of these myself but as I got a top speeds of 60 knots out of one and a prop size of 13" out of another. So I figured I should ask around. I have just been informed that my motor was split from a twin set up. I imagine running twins would require a different prop? Making mine useless for my set up.
 
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