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  1982 Newport OMC 90-HP no-VRO; Gains from Mounting Height, Propeller, and Foil Changes

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Author Topic:   1982 Newport OMC 90-HP no-VRO; Gains from Mounting Height, Propeller, and Foil Changes
alchse posted 07-06-2015 09:29 PM ET (US)   Profile for alchse   Send Email to alchse  
Here are some numbers I took down this weekend for my 1982 Newport with a 1985 Johnson 90 HP non-VRO, running a 13 x 19 propeller presumably aluminum. The engine mounting height is the lowest setting, and there is a Hydro-foil A-V plate appendage:

2000 16
2500 21
3000 26
3500 29
4000 33
4700(WOT) 39

I would say the conditions were above average for my water. I was happy with the performance, at least the power aspect to it. I didn't feel like I needed or wanted any more speed or power. So I'm a little hesitant to change something that seems to be working.

How much may I have to gain from dropping the foil, raising the engine, and getting a stainless propeller?

NewportMe posted 07-07-2015 02:15 PM ET (US)     Profile for NewportMe  Send Email to NewportMe     
All though I'm not as experienced as some who may reply, you may gain 2-3 mph on top speed. With that being said you also may gain decreased porpoising, better economy, and a better "ride or feel" to the handling of the boat at speed.


jimh posted 07-07-2015 04:58 PM ET (US)     Profile for jimh  Send Email to jimh     

I cannot predict what will happen to your 1982 Boston Whaler NEWPORT boat's performance if you change the propeller to a stainless steel propeller from an aluminum propeller, if you removed the Anti-Ventilation (A-V) plate foil appendage, and raise the engine mounting height, but, as a general rule, you can anticipate that:

--a stainless steel propeller works better than an aluminum propeller;

--the addition of an A-V plate foil appendage tends to create added drag, unless the foil is running complelely above the water; and,

--raising engine mounting height tends to reduce the amount of the gear case immersed in the water, which tends to reduce drag.

Since the general trend of the three proposed changes are all in the same direction, and that direction is toward improved performance, it seems reasonable to think that making all three changes would tend to improve performance, at least if we are to measure performance by the top speed possible.

In your narrative you express satisfaction with the performance you have now. You ask us to give you an estimate of how much you can gain. We can only express that there is some reasonable assumption that you might gain a bit of improved boat speed and perhaps some other favorable characteristics from making all these changes. Only you can know if your satisfaction will also improve. If the boat runs well now, you are satisfied, and the potential for improvement seems small--which I would say is a reasonable assessment for your boat given what it is doing now--you might gain a bit of speed but I can't say if you will gain a lot of satisfaction.

jimh posted 07-07-2015 05:03 PM ET (US)     Profile for jimh  Send Email to jimh     
Although you have not asked for this advice, I will also comment on the performance you are getting now from your preset set-up. I think you are carrying too much propeller pitch because your maximum engine speed seems low.

Check the owner's guide for your engine and find the recommend full-throttle engine speed range, then please post that information. I suspect that your c.1985 OMC 90-HP engine will recommend that its full-throttle engine speed ought to be higher than 4,700-RPM. I suspect the recommended range will be higher, perhaps around 5,500-RPM. If you can give us the actual information from OMC for your engine, and if that full-throttle range is higher, as I suspect, you can probably improve performance just by changing the propeller pitch.

Jeff posted 07-07-2015 08:02 PM ET (US)     Profile for Jeff  Send Email to Jeff     
4700 is too low for WOT RPM. Here are my numbers for my 1976 Newport with a 1974 Evinrude 85hp V4.

jimh posted 07-08-2015 09:42 AM ET (US)     Profile for jimh  Send Email to jimh     
Jeff's test result do not show his boat going remarkably faster, but adjusting the propeller pitch to get the engine full-throttle speed higher is probably going to make the engine happier and going to improve acceleration. Holding the engine down to only 4,700-RPM may be causing the engine to lug, which typically is not good.
alchse posted 07-08-2015 02:52 PM ET (US)     Profile for alchse  Send Email to alchse     
Thanks guys. Is there an order that you would perform the adjustments?

Ideally for me the prop adjustment would come last as it is the only one that requires cash outlay, but from Jeff's post it seems like I wont see the advantage of moving the engine up without the new prop.

jimh posted 07-09-2015 01:58 AM ET (US)     Profile for jimh  Send Email to jimh     
It seems to me that if the goal is to try various changes and to perform in an order that represents the least effort and least expense, then that order is:

--remove A-V plate foil appendage

--move engine mounting higher

--change propeller characteristics

dgoodhue posted 07-09-2015 07:40 AM ET (US)     Profile for dgoodhue  Send Email to dgoodhue     
I would raise the motor before removing the foil. The foil was probably installed to offset the negative characteristics of the motor being mounted too low.
ratherwhalering posted 07-09-2015 03:22 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
Remove the Hyro-foil. It is unnecessary even with an aluminum propeller and with your engine mounted in the lowest setting, it is just creating drag.

Post your new numbers. I'm guessing 5000 RPM at 40.5 MPH. Raising the engine will increase your RPM about 100RPM per hole, decrease drag and increase speed. If you keep that big aluminum prop, at some point you will start venting, lose performance, and gain nothing but RPM, so maybe just raise 1-2 holes and see how it goes.

Ultimately, you are going to need a 13.25 x 17 propeller in order to get the maximum RPM up to 5500. I don't think you will gain any top end speed over 41-42mph, but mid-range performance and engine RPM will increase with a better propeller.

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