I over propped... problem or no?

[jamrobin]

Hi All,

Been lurking here for a while but now I need an answer to a question I don't see asked.
Picked up a Tempest 176 (16.5' not counting the little swim platform) with a 4.3 OMC, and after doing ALL the maintenance items and installing smart tabs I found I was able to rev her up beyond the 4600 (was getting close to 5k) in the manual. GPS said I was maxing out around 53mph with a 14.25" 3 blade 23 pitch prop.

This is my 4th boat (some outboard, some I/Os) but my first OMC and I've always wanted to try out a 4 blade prop, so I ordered one from here... my reading suggested that if I keep similar diameter and pitch, the 4th blade should reduce my rpm by abour 300... I bought the Solas Rubex HR4 stainless 14x23.

I am VERY pleased with the performance of this prop... combined with the tabs, my hole shot is something like 1 second, it has considerably more speed throughout the majority of the rev range, it bites in turns with only a tiny down trim, but I now top out at "only" 50mph (fast enough for me) and now seem to be limited to 4k max. This is a bigger drop than I expected in the wot RPM... (could it be the cupping I didn't account for?) I've only burnt through a half tank of fuel since installing it so I don't have a really good handle on whether it's much different because I had more people in the boat over the weekend than my usual just me and a passenger is my norm.

Now the question,

Am I doing any kind of potential harm to my engine by being over propped? I do most of my boating around 3500 anyway, and only max it out once in a while... (OK almost daily but not usually for all that long ;-) )

I assume this is presenting more of a load to the engine, but is there any harm in that? I live on the (Ottawa) river and hop in the boat either for fishing, skiing, or goofing off in general almost every day that the sun is shining (and even some rainy ones)

Thanks in advance for any insight,
Jamie

 

[steelespike]

Re: I over propped... problem or no?

Welcome,
Ok heres what I get at 5000 the boat should be doing about 58 mph. at 4600 about 53.
This is assuming a 1.68 gear ratio and 10% slip. So it would seem your tach is not accurate.
If your smart tabs are too aggressive they will slow the boat both from simple drag and forcing the bow down creating more drag.
We usually suggest making one change at a time. I would fully release the tabs and see what you get with some runs
carefully adjusting the trim without the tabs.

 

[jamrobin]

Re: I over propped... problem or no?

Welcome,
Ok heres what I get at 5000 the boat should be doing about 58 mph. at 4600 about 53.
This is assuming a 1.68 gear ratio and 10% slip. So it would seem your tach is not accurate.
If your smart tabs are too aggressive they will slow the boat both from simple drag and forcing the bow down creating more drag.
We usually suggest making one change at a time. I would fully release the tabs and see what you get with some runs
carefully adjusting the trim without the tabs.

Thanks,
You're numbers seem to reflect my observations before installing the tabs.

I do indeed have a few doubts about the tach... if I bang on it it will sometimes jump up or down by a couple hundred rpm... When I tuned her up I used an external tach and dwell meter, so maybe I'll try a few runs with the doghouse open and a friend reading out the external tach... I really don't have a lot of faith in the robustness of the ground bus behind the dash, (needles all dance a bit with the stereo playing loud) so if my readings are different between the boat tach and the external one on a run, I'll pull the tach and maybe give it a dedicated ground.

It's raining cats and dogs here today, so I'll go out after work tomorrow and see what I get with tabs flapping freely, then reconnect them one setting at a time to see what results. (I did have to knock them down one stiffness level when I first installed them because they were sending spray straight up the transom... obvious drag)

The extra few mph don't concern me too much but if I'm wasting power pushing the tabs, or receiving inacurate information from the tach, I do want to sort that quickly.

 

[steelespike]

Re: I over propped... problem or no?

Let us know what you find.

 

[jamrobin]

Re: I over propped... problem or no?

Let us know what you find.

OK I tried letting the tabs flap freely and I'm happy to report that I got some of my top speed back.
I made 3 runs each up river (wnw), and and down river, (ese obviously) with a 10 mph wind from the north. (This stretch of the Ottawa is very wide, deep, and slow moving)

I was getting a fairly solid 53mph down river and 51mph up... I was struggling with how much trim to give her because I had put the tabs on 3 weeks before the prop... with the tabs disconnected and the new prop I found myself trimming well above the guage marker with no cavitation at all (or at least none I could hear), so I played with it up until the stern felt too loose, and back down until I could feel (and see on the gps) the speed go down.

I am now even more suspicious my tach is wonky... I retightened all the connections between each run and it helped but I don't know if I trust that it maxed out at EXACTLY 4k every time, and is a little erratic about getting there.

I re-attached the tabs at the lightest setting and ran 3 times each way again and I got 52 down and 50 up consistently but with the added stability and reduced hole shot and bowrise I liked from the tabs... (I'd call the combination close enough to dialed in now) Oh... for good measure, I recorded my speed at 35mph @ 3k as well... I have a little more trust in the tach at lower revs.

I definitely seemed to have had the tabs one setting too stiff.

Does anyone have a good diagnostic procedure for my tach? I'll be busy fishin' tomorrow but I could pop it out and bench test it in the house and try it out again the next day.

I also reiterate my first question... If my rpm is indeed maxing out at 4k and it's not just a pooched tach, then I'm over propped... is there any harm to my engine in that?

Thanks,
Jamie

 

[steelespike]

Re: I over propped... problem or no?

According to my info 1.68 ratio with a 23" pitch produces 53 mph at 4600 even if the 51 was accurate your within limits.
My info says 4200 to 4600. 3000 at 35 indicates about 10% slip avery reasonable number perhaps actually a little too low for 3000,but the tabs may be helping the speed and thus reducing slip.
Might be interesting to see your performance with and without the tabs at various rpm.
If you want to play around.

 

[TilliamWe]

Re: I over propped... problem or no?

I also reiterate my first question... If my rpm is indeed maxing out at 4k and it's not just a pooched tach, then I'm over propped... is there any harm to my engine in that?

Yes, there is. Valves first, pistons second.