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Old 03 January 2010, 17:47   #1
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Mariner 60HP bigfoot prop problem

Hi guys
this is my first post, and my first boat! so i hope you can advise me.

to outline,
i have a 17ft Dory Hull, with a 60HP longshaft bigfoot engine, Mariner.
the boat and engine weighs about 900kg

the engine has just been overhauled, and runs well.
i am sure it has a 13" prop. but could be wrong.

i use the boat for diving, with skipper + 4 divers.

if i am on my own, she will plane at 30knotts
but as i load the boat, logically the speed decreases, until she barely gets on the plane at about 10knotts

i hope that is enough info, to make my question valid.

will changing the prop help me get the boat faster?, if so, what would you recommend?

or am i asking too much from the engine?


thanks for any comments
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Old 03 January 2010, 18:39   #2
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Mark, Welcome to the forum.

The simple answer is you are probably asking too much of the engine . But for anyone to say for sure they would need to know what revs you currently get at full throttle and what the maximum rated revs are for the engine. (The gearbox ratio and actual prop pitch (rather than a guess - which might turn out to be the diameter would help too).

You might be able to improve things a bit when fully loaded (and possibly lose speed when lightly loaded) but you're not going to get 30 knots fully loaded if thats what you are hoping for.
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Old 04 January 2010, 00:33   #3
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Quote:
Originally Posted by markymarkP View Post

will changing the prop help me get the boat faster?, if so, what would you recommend?

or am i asking too much from the engine?
I have a lighter RIB (350-400kg) with a 60hp and can get 30knots unloaded as well that maxes me out right at 59-6000rpm. I am sure I could go faster with a steeper pitched prop but I would have an even harder time planning with 4 divers & gear aboard. I have considered getting a shallower pitched prop but when I am running with what should be my max load (which is less than what I try to put in her sometimes) I am pulling 52-5300 rpms which is in the range of where it should be.

Basically if you are getting into the lower portion of the engine's range >5000 but <6000rpm(?) for you engine fully loaded that's as good as you're going to get from the 60hp. Unloaded top end is a differernt optimum.
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Old 04 January 2010, 03:47   #4
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Mark,

Again, welcome to ribnet.

the very high level way of looking at prop pitch is to compare it to the gears on your car.

You;ll to 70mph in top, just donlt try to get moving too quickly. At the other end of the scalem, you need a low gear (1st) to start, but not mnucgh use above 10mph.

Two extremes, but the theory still holds. On an outboard you can't change the pitch when you are moving, so you need to find a happy balance. Possibly even change form one prop to antother depending whether you are alone on a cruise or have 4 mates + diving gear aboard.
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Old 04 January 2010, 11:46   #5
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On an outboard you can't change the pitch when you are moving, so you need to find a happy balance. Possibly even change form one prop to antother depending whether you are alone on a cruise or have 4 mates + diving gear aboard.
That's the plan I use (assuming I have enough advanced notice on the number of divers.)

I use a 13.75 x 15 SS prop normally, but if it sounds like I'm going to have 6 or 7 (total) divers onboard, I swap to a 13.5 x 15 aluminum (more flex + less weight = higher rpm.) If it's a heavy load and heavy seas where I don't need top speed, I have a some diameter x 13" prop on hand, but I've never had to use it (and to be honest, usually don't have it with me...)

jky
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Old 05 January 2010, 11:03   #6
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thanks a lot for all your replies
much appreciated
you have given me much to think about.

perhaps i should just get a bigger engine, that way i can use the boat at its best, rather than straining it to get the max from the engine.

on that basis, what size engine? 120HP ?
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Old 05 January 2010, 13:11   #7
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For diving loads you want the largest engine that doesn't exceed the capacity of the boat.
For a 17ft Dory I'd be surprised if it was rated for as high as 120hp. 90hp sounds like a more typical max allowable engine size for the type.
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Old 05 January 2010, 13:23   #8
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I vote with Captain Jack.

Check the max HP (and check max weight as they are not necessarily the same especially if looking at 4 st vs 2 st) you can put on and go with that. My SR was ok with a 60 before but with the 90 she is a lot more fun and a lot more capable. I have also found I actually use less fuel as I am not running the engine maxed out but throttled back and hence much more fuel efficient.

Ian
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Old 05 January 2010, 13:30   #9
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Thanks for the reply
it's a 17ft rigid raider, used as a harbour masters boat in a previous life, so it's very strong with heavy thick glassing
I am waiting until my local boatyard reopens to confirm the transom will take such a beast, but if it will, it may be the answer
I suppose if it's too big and I fitted such an engine , the boat could break??
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Old 05 January 2010, 13:34   #10
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Yeah having the transom crack or separate from the boat is not good for business.

Even 90hp would be 50% more umph though. As long as you don't try to put 50% more diving stuff aboard you'd be alot more capable.

I can carry 4 (small) divers and 8 tanks on my 16ft RIB with 60hp Yam. But as soon as I add a 5th person as a tender I am not planing anymore.
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