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Discussion Starter · #1 ·
Hi everyone,

I am new to boat owning and have a CJR 14 with a 1985 35hp Mercury engine. I had it out on Loch Etive last weekend, and have a few questions about propellers if anyone can help me.

I admit I am a bit baffled as to all the talk of propeller pitch etc, but I understand the theory of ventilation and cavitation, a theory I put into practice by chewing my prop on some rocks. It certainly led to much vibration and churning of water without much movement! It is now with Steel Developments for repair.

Before I damaged it however, I did get a wee shot in the boat by myself, and was pleased at how it went. What I did notice though was that at full throttle, especially if I put the power on quickly, it seemed to just spin too quickly to move the boat, which came offthe plane and slowed down. If I applied power more gradually, I could get it up to almost full throttle and quite fast too, but the last wee 1/5th of throttle would lead to it spinning too fast again.

After a bit of research, I am aware that changing to a differently pitched prop could alleviate this, but was baffled by the talk of obtaining max throttle and RPM ratings. I dont even know what pitch my prop is, although I have asked Steel Developments to advise on suitatble replacement props for my engine. Hopefully they will tell me what my current prop is!

Could anyone tell me the laymens terms pitch thoery on how to match the best prop for the engine, I dont want to tow waterskiers or accelarate like a Ferrari or anything, just want to be able to go to full speed smoothly now and again without turning the water into a cappucino!

Also, Doel fins, I heard good reports on these and as they are relatively cheap, I bought some to fit to the engine when I get the prop back. Would like to know the views of others with these, did you notice a difference? My engine does not have power trim, just the pin and 5 settings, I stared on the second hole, but thought with three aboard that the engine was sitting down too low, so now have it on the middle holes. Will experiment later when I have a prop!

Be good to hear any advice you have, keep technicality to a reasonable level please!
 
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I cant help with what will be correct pitch prop for your boat but this pitch calculator from Castle Marine will work it out for you if you know all the information that it asks.
I tried several different props before I found the best one for my boat which incidentally came from Steel Developments. I tried a 17", 16" and 15" without success until they suggested a 15" but 1/4" smaller diameter. This made all the difference.
As for the Doel Fins, I have tried them and although it helped the boat get on the plane quicker I reckon it cost me a couple of knots top speed and added 20% to the fuel consumption so have now taken them off. If you get the correct prop I dont think you will need them.
 

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I am also considering adding Doel fins. My boat is a Ryds 430 with a 30hp on the back. The problem is that the console is towards the rear of the boat meaning I am sat at the back of the boat (17st). The outboard fuel is in the rear storage locker and I have an Auxilary o/b on the back. When I open the throttle, unless I have someone sitting right at the front, the stern will bury and it is difficult to get on the plane.

My question is, will doel fins help by assisting in lifting the stern earlier and leveling the boat. Answers much appreciated.
 

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I am also considering adding Doel fins. My boat is a Ryds 430 with a 30hp on the back.

My question is, will doel fins help by assisting in lifting the stern earlier and leveling the boat. Answers much appreciated.
In theory ,yes.

they can benefit by `lifting` the stern a bit quicker to help you get out of the `hole` and up on to the plane.

BUT, be very aware they can cause `interesting` handling characteristics.

I tried them on a 19ft Orkney Fastliner, imo very dangerous.
If the boat was steered either way when under way at planing speed when the steering was returned to straight ahead the boat would remain heeled over and still steering round the corner. In effect the steering was completely lost until the boat came off the plane. Very dangerous.

Ask yourself the question, why don`t engine manufacturers who spend millions on R & D make the cavitation plates that the fins bolt to 2 ft wide in the first place if that was the answer ?


Most people try them then bin them.



Dave.
www.kentcharterfishing.com
 

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Before I damaged it however, I did get a wee shot in the boat by myself, and was pleased at how it went. What I did notice though was that at full throttle, especially if I put the power on quickly, it seemed to just spin too quickly to move the boat, which came offthe plane and slowed down. If I applied power more gradually, I could get it up to almost full throttle and quite fast too, but the last wee 1/5th of throttle would lead to it spinning too fast again.
I'd wait til you get your prop back from Steel Devs before adding the fins or speculating. It sounds like you might have had a spun prop hub (the rubber bit in the middle). You may find you have a big improvement when the prop comes back.
 

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I'd wait til you get your prop back from Steel Devs before adding the fins or speculating. It sounds like you might have had a spun prop hub (the rubber bit in the middle). You may find you have a big improvement when the prop comes back.
Just what I was thinking too.

Happened to me a couple of years ago, I thought the prop had dropped off!:headhurt:
 
G

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This is unlikely to be the case, but a consideration, but the paragraph the guys have just highlighted could also suggest a short shaft engine on a boat requiring a long shaft?...
 

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Discussion Starter · #8 ·
This is unlikely to be the case, but a consideration, but the paragraph the guys have just highlighted could also suggest a short shaft engine on a boat requiring a long shaft?...
Thanks for all the replies, I will certainly be wary of the Doel fins, and will firstly try things out without them when I get my prop back.

I have enclosed a pic of the engine on the back of my boat, and with reference to the above comment Tom, I am pretty sure this isn't a short shaft engine and is the right size for the boat. Feel free to point out if I am wrong though!!!!!!!

Cheers

Fergusdaddy
 

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... from that angle it looks OK. Cavitation plate is a gnats wotsit beneath the bottom of the boat so shouldn't be causing the "slipping".
 

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Could the engine be mounted too high on the transom?
Do you have a picture from the side at the height of the cavitation plate so we can see what height it is in relation to the bottom of the hull? It needs to be roughly 25 mm below the hull if i remember rightly?

Also could the prop be under-pitched?
 

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I won"t get into the doelfin pros & cons, personally I like them. The engine height from your pic looks fine. It could well be a pitch problem. Also if the boat was struggling to get on the plane & you moved the trim pin out you would just of made the situation worse with the boat being backend down nose up & the prop spinning. It should of been trimmed in a notch or weight moved forward or a combination of both. IMHO

headlight
 

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Discussion Starter · #12 ·
Could the engine be mounted too high on the transom?
Do you have a picture from the side at the height of the cavitation plate so we can see what height it is in relation to the bottom of the hull? It needs to be roughly 25 mm below the hull if i remember rightly?

Also could the prop be under-pitched?
Don't have a picture that low, will take one and post it tomorrow, although the cavitation plate is around that distance below the underside of the boat. The prop may be under pitched (?????) but I will wait and see how it performs when Steel Developments sends it back.

As for the trim pins, I thought the engine and boat were supposed to be on the same level plane, and it looked to me that he engine was sitting lower at the back than the front in pin hole 2 (i.e the second hole out from the transom) so I moved it out to hole 3, you can see it running in hole 3 in the first photo on my post "Etive 26th July" on the Scotland page, must be around 5 pages deep by now though!

Like I say, I am new to boat owning, and will be taking it out on the river Forth near where I live when the prop is returned for "sea trials" with just me aboard and at all trim angles etc until I get used to it. Then, and only then, will I try the Doel fins.

Thanks again for all the advice, and I will keep you posted here!
 

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Discussion Starter · #13 ·
As you can see in this photo, the caviatation plate is the same level as the lowest point on the hull. Also, with regards to trim angle, the transom on a CJR isn't vertical, it leand out towards the top, but I have treid to keep the engine sitting vertically by moving the trim pin out.

Will keep you posted as to my experiments!
 

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FergusD.

As others have said I really wouldn't worry about this problem until you have feedback from Steel Dev about the prop..... you need to knopw its dia/pitch and for them to have tested the hub isn't slipping. They should test it anyway but did you mention it?

Have a work through that propcalc program a few times, you will need to know the gear ratio of your outboard though. Also do not underestimate the weight of your boat, outboard, fuel, people, misc kit.

When I ran it for my boat recently (to buy a new prop from Steel Dev) a totally different prop would have been needed for a stripped out boat one up compared with laden and 4 people.

I went for the laden prop which drops about 4kts max over the alternative.

Finally get to know your boat without the fins first... at speed and in turns/harsh useage. Then add the fins and carefully explore their effect.

David
 
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Hi Fergus

Get the prop sorted first, but you will almost definitely want to drop back to pin position 2 or even one. The boat is trimmed way out like that and all that's going to happen is you'll end up lifting the bow rather than optimising forward motion. If it is possible you may also want to drop the engine by about another half an inch to an inch and that will give you more grip in turns and as you climb onto the plane. I think you are cavitating a bit at the moment and air is getting under the cavitation plate rather than being forced over it and you only want water under it.

Tom
 

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Discussion Starter · #16 ·
Cheers guys, will heed your expert advice and let you know how it goes!

I don't think the engine can sit any lower but I will check.
 

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Tom is agreeing with my ealier post. If you are struggling to get on the plane you need to trim the engine in not out. This a common problem with manual trim engines. With power trim you pull off with engine trimmed in & then as the boat comes on the plane trim out. With a fixed trim it"s going to be a compromise. The best setting for planining may be 2 holes out but the boat may struggle to get on the plane in the first place at that setting. Weight distribution on the boat will affect the trim. It may be that you will need to get people to move to the front of the boat ( assuming you have people on board ) to alter the weight distribution to get on the plane then once planing have them move back.
Hope this makes sense.

headlight
 

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Discussion Starter · #18 ·
Tom is agreeing with my ealier post. If you are struggling to get on the plane you need to trim the engine in not out. This a common problem with manual trim engines. With power trim you pull off with engine trimmed in & then as the boat comes on the plane trim out. With a fixed trim it"s going to be a compromise. The best setting for planining may be 2 holes out but the boat may struggle to get on the plane in the first place at that setting. Weight distribution on the boat will affect the trim. It may be that you will need to get people to move to the front of the boat ( assuming you have people on board ) to alter the weight distribution to get on the plane then once planing have them move back.
Hope this makes sense.

headlight

I need thinner fishing buddies! 50 stone of Scotsmen is more than enough ballast for any boat!
 

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