need opinion please

[BruceDeuce]

I have a 1990 70hp Johnson VRO on a 16' Sea Nymph boat weighs 900lbs without motor. I don not have access to the boat at the moment but Im trying to put together the parts I need to make it ship shape. I need a prop and they range from 9" to 21"pitch. I like to get out to the fishing spots as quick as I can but I also like to troll. What size /pitch do you recommend?

 

[steelespike]

Re: need opinion please

Prop selection is heavily dependent on your present prop size and wot rpm and gps speed.
Your prop size is usually near the base of the blades or under the prop nut.
I can tell you that the prop for best speed and the prop for trolling are very likely at opposite ends of the spectrum.
If you want to troll with your 70 you will likely need a trolling plate or a drift sock.Something to slow the boat.
Its possible that if your a dedicated troller you may need a kicker motor.
As far as speed; attention to the details of your setup may improve your speed.
Eliminate as much weight as possible.Weight placement is important.
The antiventplate (just above the prop) should be even with or slightly above the bottom.
Be sure the motor is in excellent tune and the throttle opens all the way.
Bottom clean and smooth.
What year is the Sea Nymph? Aluminum or Glass? Do you know the present prop size rpm and gps speed?

 

[BruceDeuce]

Re: need opinion please

Its a 1990 aluminum, the motor came with the boat. All original. Im not able to get to the boat at this current time. I just purchased the boat and haven't had it out on the water. I dont know the prop size at this point. Your probably right i need a kicker. Pics of my boat can be seen here. http://forums.iboats.com/sea-nymph-...eanymph-boat-owners-437708-2.html#post4033013

 

[jay_merrill]

Re: need opinion please

You have mentioned the empty weight of your boat, but do you know what it weighs with the motor and your typical load of fuel, gear and people?

For example, your motor weighs about 250 pounds and 20 gallons of fuel weighs about 120 pounds. This being the case, your boat, motor and fuel weighs about 1,270 pounds. To this, you must add the weight of your anchor, PFDs, first aid kit, flares, cushions, cooler (with ice), etc. Before you even put any people in the boat, you are probably up to about 1,500 pounds.

Given the resulting power to weight ratio, I would guess that you need a 13-3/4 wheel of about 15" to 17" in pitch. I am basing this assumption on once owning a boat of similar weight, with a 65hp Johnson triple on it. I was probably a little heavier than your boat and ran 29 statute MPH (by GPS) with a stainless Powertech prop that was 13-3/4" by 15". I was showing 5,100 rpm at WOT, with this configuration.

With a little bit lighter boat and 5 more HP, you might get away with a 17" prop, but if you can't go that high, I would expect speeds in the lower 30s, with a 15" pitch prop. Your WOT rpm should be right at 5,500 (the upper limit of the recommended WOT rpm), or a little above it.

BTW, your recommended WOT rpm range is probably 4,500 to 5,500. Double check me by looking at your manual (if you have one), but I think that's been the correct range for the OMC triples through many years. If you are looking to prop the engine to reach the 5,500 rpm point at WOT, which is what many people recommend, plan on doing that at your typical operating weight. That way, if you load up a little beyond the norm, you will not fall below the lower end of the acceptable WOT range (4,500 rpm) and end up operating at high manifold pressures (lugging the motor). Lugging your motor for prolonged periods of time is not good for it in general, and tends to load your spark plugs and piston domes with a lot of soot, carbon, etc.

I hope all of this helps.

 

[BruceDeuce]

Re: need opinion please

Thank you for a very extensive explanation. I do have a Seloc manual, but I cant find anything about WOT. I will be able to get to the boat on Thursday so i will Identify the Prop and list here. With your breakdown, I believe I would need to be able to push atleast 2200 lbs. and that would be give or take on any given day. And with that weight you suggest a 17'' pitch, correct?

 

[jay_merrill]

Re: need opinion please

At 2,200 pounds, you'll probably need the 15" pitch prop. That's pretty close to my usual operating weight in the boat I mentioned and 17" was too much for it.

BTW, I looked up your boat, just to get more familiar with it. One thing that I noticed right off, is that most people seem to operate them with about 115 to 140 hp on the back. Those boats will go a bit faster than your (lots faster with 140 hp), but they will probably burn more fuel. My 65 hp Johnson was pretty economical, at least for an older motor. I checked fuel burn by running a fixed course one day (I plotted it in Google Earth) and figured that I was getting about 3.5 miles per gallon. I wanted to know that number, because unlike most boating situations, how far you can go on available fuel is more important around my area, than gallons per hour of consumption.

 

[BruceDeuce]

Re: need opinion please

Yes Ive seen these boats with bigger motors. I will be happy if I were to get 35mph out of my configuration. 3.5 mpg is good and I would be happy with that. My actual operating weight would be approx. 1800lbs. The 2200lbs as stated would be for when I take the boys out, which would only be a few times a season. Would the 15'' pitch be appropriate for 1800lbs as well? What about 3 or 4 blades? Make a difference?

 

[steelespike]

Re: need opinion please

My info indicates 5,000 to 6000 rpm range.
if you can turn a 15" prop up to 6,000 the prop calculator indicates 31.7 mph at 10% slip.
A 3 blade prop is usually considered to deliver good overall performance with the best top end.
A 4 blade can have better hole shot,stay on plane better at reduced throttle,resist venting,seem smoother,
handle better at speed and around the dock. Usually delivers slightly less top end.

 

[BruceDeuce]

Re: need opinion please

Steel and Jay certainly know your way around boats! Thank you both for the wealth of info given to me. Please tell me what is "resist venting''? And if I were to go with a 4 blade, what size do you think I should use. I want best all around performance. Thanks Bruce

 

[steelespike]

Re: need opinion please

In the old days we called it cavitation.But cavitation you don't feel venting is when the prop
loses grip usually because its fouled with air.
Usually it is suggested to drop one inch in pitch when going to a 4 blade prop. But we don't know for sure if the 15
is what you need. I think I would stick with the 3 blade. And think about a 4 blade if the 15 isn't quite right.

 

[BruceDeuce]

Re: need opinion please

I will certainly try the 15'' pitch prop. I heard that some places let you try props before you buy them, Im not certain of that. But I will look into it and start with a 15'' then try a compatible 4 blade. Thank you for all your help. Bruce

 

[BruceDeuce]

Re: need opinion please

I was able to get to the Boat yesterday and the prop is 13-1/4X17'' pitch. My WOT tops out at 6000.
The previous owner said he was happy with this configuration, but only ran ran this in the rivers. Not often at WOT. I would really like to go to a 4 blade for better handling and less vibration.

 

[jay_merrill]

Re: need opinion please

I'd stick with the 3 blade. Unless you are planning on running on the edge of plane, I don't think the 4 blade will give you "better handling." I also don't think you'd notice much difference in vibration.

Also, if you are topping out at 6,000 rpm with just yourself and typical gear, and that is the correct upper rpm limit (SS knows his stuff, so I'll take his word for it) for your motor, I think you have the right diameter/pitch configuration. Adding a couple of hundred more pounds for kids, etc., isn't going to drop your rpm below the recommended floor of your rpm range.

I recommend that you give this configuration a try for awhile and see how your performance is in a variety of loads/conditions.

 

[BruceDeuce]

Re: need opinion please

Thanks for the response Jay! I will try the current prop and see how it goes. Its starting to get cold here, so it may be a few months before I get it into the water. I will post up my results when I do!

 

[steelespike]

Re: need opinion please

If its a 17" prop and your turning 6000 assuming 10% slip your speed should be about 35.
In the spring you should try a light weight run and a typical weight run and get your rpm and gps speed.

 

[BruceDeuce]

Re: need opinion please

If its a 17" prop and your turning 6000 assuming 10% slip your speed should be about 35.
In the spring you should try a light weight run and a typical weight run and get your rpm and gps speed.

Will do thanks!