Prop Sizing

[Capt Paul]

I need to vent a little...

I need to change the prop on my boat. When I hauled out for bottom painting last week I had a local prop guy take a look at the situation. He agreed as did others that it was time to change the prop. He told me he needed to get the old prop off to look at the numbers on it to properly order the replacement prop.

New prop comes in and goes on the boat. I take the boat out and its "singing". Not a huge problem and can be fixed. Big problem then comes up. I bring boat up to cruising RPM and I'm only doing 9kts where I usually do 15 kts. Prop guy says don't worry I can fix the problem buy adjusting the prop. Something doesn't sound right to me. Can a prop supposedly ordered via the spec of the old prop be that far off? Can he really adjust this prop properly so that I get the same performance?

Any experiences you can share will be appreciated. I have never had to replace a prop on a boat before. Being and engineer I cam completely baffled buy this.

Thanks,

Paul

[Mariner]

Why did the old prop need to be replaced?

If you're only getting 9 knots, something is very wrong. Any modifications, as well as yard fees to have it hauled should be at the expense of your prop guy.

[DougSea]

To quote Mariner, something is VERY wrong with the new prop. There is no way you should have lost 6 knots. That's multiple inches of pitch wrong.

What engine / transmission setup do you have? The guys on the board were very helpful when I had to have my prop re-done (due to a prop strike). Post your info here and I'm sure the board will help you out.

Good luck! (And consider finding a new prop guy!!)

[Capt Paul]

Doug and Mariner ,

Thanks for your input. I have the Yanmar 6LP-STE 300 HP with a Velvet Drive 5000 Series transmission. The prop needed replacement because it was pretty well all pitted up. I did recently strike something also. Speaking with the prop guy he has guranteed me he will make this work. I just hope it is not one of those adjust an experiment kind of deals that goes on for two weeks. I am already kicking myself for not working this through my mechanic who always come through for me.

Paul

[DougSea]

You should check these guys out:

http://www.thomasmarinepropeller.com/

They're the local "Prop Scan" company in your area. Well, the closest anyway. It's a franchise business and the guy in Stamford CT just did prop reworks for Dennis and myself.

I don't have my boat back in the water yet so I can't give you a real-world confirmation yet that the work will be as good as claimed but going on the testimonials and looking over my prop I'm feeling pretty confident. I don't think Dennis is back in the water yet either but I'm sure we'll hear from him once he's splashed.

One of the nice things about having them work on your prop is that they store the exact details of your current prop and any Prop Scan shop in the country can then replicate those settings in a new prop.

[Merry Kate]

Capt Paul - Amen to what DougSea says. Take it to a certified PropScan shop. They do not need to read any numbers off the hub - they computer scan every parameter of every blade. There is a lot more to props than diameter & pitch. Cup, DAR, etc all dramatically affect performance and loading.

A PropScan shop can dial it in to within 20 rpm of where you need to be, which is turning 50 to 100 rpm OVER your rated engine speed at WOT with a full load of fuel, water, and gear. Just be prepared to give them all of the performance numbers achieved with the old prop when you take it in. It'll be the best $200 (or so?) you've ever spent on your boat.

You didn’t say: were you making 9 kts instead of 15 because of too little pitch (i.e. the motor would easily rev all the way past rated speed) or because of too much (i.e. black smoke and the motor wouldn’t rev anywhere near rated speed)? If it’s due to overload, I can't emphasize enough; DO NOT run your boat any more with a prop that is loading your engine that severely. Good luck and don’t worry – the right prop shop can get you fixed up perfect the 1st try.

[Capt Paul]

Merry Kate,

Thanks for your words of encouragement. I was not getting any black smoke. The engine was very responsive and reved right up to my cruising RPM of about 3100-3200 RPM. This is where I'm usually doing about 15kts. So it sounds like to little pitch as you say. I did not try WOT as I was not very comfortable with what was happening. At WOT I should be at 3800 RPM. I am actually going to try this tomorrow morning weather permitting.

Just to be sure, are you saying I should have my old prop scanned or the new one that is not tuned properly? I’m guessing the new one needs to be analyzed.

[DougSea]

If you had decent performance out of your old prop I'd be having that one scanned - that would establish your baseline.

Then I'd make sure the new one matched that. As Merry Kate pointed out the numbers on the hub don't mean anything - you'd want to know what you were actually spinning for a prop, sounds like the new one was seriously under pitched!

Let us know how this all turns out. Good luck!

[jleonard]

Just to be sure, are you saying I should have my old prop scanned or the new one that is not tuned properly? I’m guessing the new one needs to be analyzed.

Yes get this done. All the prop shops around here will scan free as it only takes a couple of minutes.

[Merry Kate]

Capt Paul - Yes, take the old one to be scanned, as that is your baseline. Be prepared to tell them what your performance was with it, the most important number being the rpm you turned at full throttle. If 3800 rpm is your engine's rated speed, you should be OK matching the old one (I'm real conservative about engine loading and prop to turn 100 rpm over rated).

If your new prop is that under-pitched, go boating in the meantime with no worry at all - just a lot slower than usual!

[Capt Paul]

Well,

I took Antoinette out today with the prop guy in tow. Showed him what I was getting at 3100 RPM and then went to WOT. We reached about 3850 before the governor kicked in. With the last prop at WOT I could reach about 3650 (Assuming my TAC is correct). He said what he installed is a 19x16- 4 blade prop. I pretty sure the old prop is a 19x19- 4 blade with cups. I gave him the performance numbers. I'm planning on getting my old prop scanned, but he wanted to try and do it without that first. So we will see. As soon as he puts his revised version on I will let you all know the outcome.

I was on boatdiesel.com and noticed that another member has the same engine and transmission as mine. His specs also show that he has a 19x19 4-blade prop with cups. Assuming his transmission ratio is the same as mine 2:1 it all starts to make some sense.


Paul

[Capt Paul]

Hello all,


Well I took all of your inputs and took my old prop to the local prop scan place. He ordered me a new prop and tuned it up using PROPScan. Verdict is : EXCELLENT!!!! Actually looks like I may have picked up a knot or more. If you look at my last post on the subject it was April 19, it took a month and a half to sort through all this with the other prop guy. He tried to tune the prop but it did not change the performance. I decided to work with Thomas Marine Propeller( the local Propscan place). Great service and he understands propellers very well. First shot, he got it right and a decent price.

Know it's time to go fishing!

Thanks again for all the input!

Paul

[DougSea]

Hey Capt Paul,

Glad to hear the Prop Scan path worked out for you. I know I'm a true "convert" and any inboard boat I own in the future will have it's props scanned and tuned. My boat is running faster and turning the proper rpm's for the first time since I bought her last year. And that's after one pass through the local prop scan shop, no messing around with test runs and adjustments!

[diversion]

Your prop guy sold you the wrong prop. You cannot adjust pitch enough to make up the difference in performance that you are talking about. Pitch can only be adjusted within a certain range as set by the manufacturer. If you go to the Michigan Wheel websit, you can put your boat specs in and it will tell which prop you should have.

[Capt Paul]

Yup, I agree with you, The prop he gave gave me was a 19X15 and he tried to adjust the pitch to make up for it. It should have been a 19X19. I didn't find this out unitil yesterday when I was there for the switching of the props. Not sure why he thought this would work. I'm just glad this is a done deal..