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Prop size for 4-107

6K views 22 replies 6 participants last post by  lancelot9898 
#1 ·
Anybody have any idea what prop size would be correct for a Perkins 4-107 in a 41' Gulfstar. From what I can tell the 4-107 should have an rpm of 3000, but I can only get 1800. I have a 16x16 prop and I am wondering if it is wrong. Been running this way for years and seems ok, I can only get about 5 knots out of it. I always thought that was all I would get. But, I started to wonder ifI the prop my be an issue and I wanted to see if anyone might know.
 
#3 ·
I have the 4-108 on a Tayana 37, which is smaller than your Gulfstar but probably a bigger displacement. It came with the 17x11 3 blade, however from day one I switched over to the 17 inch max prop. The max prop has flat blades so the efficiency is not as great as the curved blades of a fixed prop so the pitch will be somewhat different. I played around with the pitch and when I matched the pitch to the original prop I was not happy with the performance. I wanted more speed at a lower rpm. For the last twenty plus years I've have the prop set at around the 14 inch pitch mark. The rpm of the 4-108 is rated at 3600 continous and I can not achieve that with the 14 inch pitch. I'm happy to cruise at hull speed doing 2500 rpm which is 80% of what my max achieveable rpm under that 14 inch pitch. Most people believe that you should be able to achieve the max rpm rating of your engine....which I can not.
 
#4 ·
When I try and push the engine above 1800, it starts to smoke a little and then the temperature starts to climb. Not necessarily to overheat, just higher. I usually will do that only if I need to get more speed for a short distance, like trying to make a drawbridge are something. But, for the most part the engine likes running at 1800. If I change the prop to get higher rpms will that effect the engine now that it has gotten use to operating at the speed it does?
 
#5 ·
Smoking and overheating are signs of too much prop. You probably se some black oil in your exhaist also, right? Change the prop to an 18 by 12 and you should be fine. If you prefer the performance of a three blade 15 by 10 will work and give you more tip clearance for less vibration. You will be doing your engine a a favor by doing this.
 
#6 ·
Check the tachometer as well. The tach on my boat is shot and reports 2000 rpm when the engine is actually turning 2800 rpm. I tested it using a digital tach measuring a point on the flywheel.

I've adapted, but I should also replace the tach someday.
 
#8 ·
I have checked the rpm's with a digital tach, it it appears fine. However, I do have a problem when my batteries are full, the alternator doesn't put out so my tack reading is off until I turn on a blower or something.

I guess a side question would be, anyone know how to get the tach reading from the engine and not the alternator?
 
#9 · (Edited)
I don't know what gear redcution you have but a 16 by 16 three blade sounds like way too much prop. On the V-40's we found a 15 by 10 three blade (as I keep recommending) worked best. I think the reduct ws 2.57 to 1 as I recall. You can go with a two blade prop at the size I have already recommended but I don't think that will "fix" prop walk but it will give you the engine rpms you need and good boat speed. Your excessive prop walk may have to do with all that pitch you now have. What you have now is what we call a "square prop" i.e. diameter and pitch the same and they seldom work well.
 
#10 ·
I don't know what gear redcution you have but a 16 by 16 three blade sounds like way too much prop. On the V-40's we found a 15 by 10 three blade (as I keep recommending) worked best. I think the reduct ws 2.57 to 1 as I recall. You can go with a two blade prop at the size I have already recommended but I don't think that will "fix" anything but it will give you the engine rpms you need and good boat speed. Your excessive prop walk may have to do with all that pitch you now have. What you have now is what we call a "square prop" and they seldom work well.
Gotcha, thanks for the info I will see what I can do to find a prop matching what you say.

Also, the ratio is 2:51 to 1.
 
#15 ·
JonB:
I don't know about "square" sail plans. I'm not sure I would go along with that theory.
But for props diameter is the more important number so it figures you should give up pitch for more diameter. You size or diameter first then pick the pitch.

The way I size a prop is a method I have used very succesfully for many years. I pick up he phone and I dial:425-355-3669

This is the phone number for Max-Prop. I ask for Fred or Fredrique. They both know props well. Then I do exactly what they tell me. It never fails.
 
#18 ·
JonB:
I don't know about "square" sail plans. I'm not sure I would go along with that theory.
Interesting. It was something I read years ago and it was not explained, just one of those "That's just the way it is, nobody knows why" kind of comments.

Just out of curiosity, have you ever designed a "square" sail plan?
 
#16 ·
Interesting new development since the subject of resizing a prop came up. When I originally looked at the prop it had 16x16 stamped on the outer hub, but the inner hub had a 16x16 stamped but the last 6 was vague. It was still a 6 but it had scratch marks. Wen the subject of resizing came up I went to look again, still hard to make out, so I went to an old survey and there I found that the surveyor indicated a 16x12 prop. I went back the prop and took a wire brush to it and I can now see a faint stamp of 2 over the last six.

So, I am now assuming that the prop is a 16x16 re-sized to a 16x12. Have any idea how that effects things?
 
#17 ·
Ok we know a few things:
I figured a 2.57 to 1 reduct gear and you have a 2.51 to 1, close enough.
Your prop is either a 16 by 16 or a 16 by 12.
16 by 12 is much close to my recommended 15 by 10 than 16 by 16.
But while it is clearly still too much prop I'm not sure that it is big enough to cause the problem that you are experiencing. But it is obvious by your symptoms that you are over propped.

Find a reputable prop shop. They can measure and tell you excatly what the dia and pitch are of your existing prop. Ask them if they can resize the prop to 15 by 10. Try that. Or buy a new three blade.

If you really love your boat and you have a birthday coming up, or better yet, your wife has a birthday coming up, giver her a new Max-Prop for her birthday. It's a lot like jewelry. She'll love it. Max-Prop will send you the new prop, already adjusted for what they know to be right for your engine and gearbox. You will love it and you will have far better performance in reverse due to the syrmetrical blade design of the Max-Prop.

And it all ended happily ever after.
 
#20 ·
Here is what I plan to do; the bottom paint I use works really well, except for the prop and shaft. I assume it is because I don't change the zincs soon enough, the bottom paint wears off quicker on the metal than the hull. So, I have always wanted a second prop so that I could switch them off mid way through the year. Grind and paint the second one and have it put on. So, switch props as they get more difficult to keep clean.

If I buy a second used prop, 15x10, I will have one to switch on and off to clean and repaint. There is a local boat yard that has lots of used props for sale.

Besides, I can tell you right now, the prop as jewelry idea, it ain't going to fly with my wife. So, this is the best I can come up with for now. But, now that you mention it, she wants a Electro Scan waste system, so I can used that and claim it's like jewelery. Works for me. :)
 
#19 ·
JonB;
I never really thought about it. I design the rig prportions by eye with a target SA/D in mind. General rig proportions will be a fucntion of the type of boat I am after and the position of the keel.

So,,,I pulled out my own book and went through it looking at th sloop rigs I have designed. In almost every case of a masthead rig the fore triangle is much bigger than mainsail area. The closest I found to a "square rig" was, MOBISLE, LOON and WHITE EAGLE. They are not strictly square but they are close.
 

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#22 ·
P,

FWIW, it's generally considered a bad idea to use bottom paint on your underwater running gear (i.e., prop & shaft.) It may seem a little counter intuitive, but you'd be much better off leaving the metal bare than coating it with anti-fouling paint.

Why? The copper in the paint doesn't play well with the bronze and stainless. Underwater -- particularly saltwater -- you are establishing a small galvanic cell that will leech the zinc out of your bronze and accelerate corrosion of your stainless. Take a little time to read up on galvanic corrosion; it's well worth your time.

There are products specifically designed for props and shafts, but they can be pretty spendy. PropSpeed gets lots of great reviews, but get ready for sticker shock. Do some internet window shopping for underwater metal paint to get a better idea of the alternatives.

Personally, I'd recommend just leaving the stuff bare. If you use the boat often, just running the engine does a pretty good job of keeping the metals relatively clean. If you don't use it often, then giving the prop a coat of anhydrous lanolin before you splash will help some.
 
#23 ·
I've been using a hard bottom paint such as tridad on my max prop for over 20 years with pretty good results. However, it's important to prep the prop properly before application. Besides removing all the prior paint from the prop and getting it nice and shiney again, I etch it prior to putting the underwater metal primer(thin coats) and then two thin coats of the bottom paint. It takes me almost a full day doing this although I'm also taking the prop apart and repacking with grease as part this ordeal. I'm able to keep the boat in the water 2 to 3 years between haul outs and maybe dive once on the boat during that time to remove a few barnicles from the prop. Good Luck in getting the right prop.
 
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