prop advice

grayclan

Member
New to me 16 foot hewes craft sea runner. Original prop frozen. so I ground it off.Original owner used a 13.25x17 prop and got 30 mph loaded or not.iv'e got an old 15 spline 21 inch. Probably too. much pitch but not sure. boat will average about 2500 pounds. I'm adding a tach as it's a must. Just trying to narrow the gap between props. Thanks
 
New to me 16 foot hewes craft sea runner. Original prop frozen. so I ground it off.Original owner used a 13.25x17 prop and got 30 mph loaded or not.iv'e got an old 15 spline 21 inch. Probably too. much pitch but not sure. boat will average about 2500 pounds. I'm adding a tach as it's a must. Just trying to narrow the gap between props. Thanks

GrayClan , the best way to assess a props performance is mount it and get out on the water. Your WOT (wide open throttle) should match the max rpm rating for your motor. The max rpm should be stamped on the motor. Overpropping puts a huge load on your gearcase and other components. Hope that helps.

beemer
 
anytime you change the pitch on your prop you will either gain or loose RPM... eg....i had a 17 pitch prop, went to a 15 and gained 500 RPM, allowing my motor to run at its specified wide open throttle rpms....your roughly gain/ loose around 200 RPM per 1 inch pitch change

If you want to gain RPM you go down in pitch

If you want to loose RPM you go up in pitch...

I would say that if you went from a 17 to a 21 pitch you will definetly be loosing too much RPM and bogging down your motor...
 
New to me 16 foot hewes craft sea runner. Original prop frozen. so I ground it off.Original owner used a 13.25x17 prop and got 30 mph loaded or not.iv'e got an old 15 spline 21 inch. Probably too. much pitch but not sure. boat will average about 2500 pounds. I'm adding a tach as it's a must. Just trying to narrow the gap between props. Thanks

Every boat/motor combination is different but taking a prop up 4 inches in pitch is a big jump. You can select a propeller for the hole shot, for the top speed, or for the maximum efficiency at cruise power. What you cannot get is a propeller that will offer the best of all 3. By maximizing any one characteristic, you will usually detract from one or more of the rest. If you only use your boat for one purpose - fishing, you may want to focus only on that particular thing.

On my outboard the WOT range is 5000 - 6000 rpm. I tried 3 props and chose an every-day, 19" pitch prop for cruising efficiently to the fishing grounds. It doesn't pop out of the hole or run up to top-speed at the top of the WOT range - the 17" pitched prop would. I just wanted the prop to get the boat on plane smoothly and to cruise easily - the 19" does. The lower rpm from the 19 inch pitch saves me some money in fuel. The 21 inch prop had the rpms too low when the boat was loaded and they tell me it's less efficient.

In the end, it's trial and error and compromise but talking to a good prop shop may help in your decision-making.
 
Sorry Lorne I thought I put in that it is a 75 merc 2 stroke. 2000. I tried Hewescraft and they said they don't have the info on the combos that far back. I have a tach from an older boat that I will install and figure it out. I just thought that there must be lots of guys that have already played the game with similar sized boats. Thanks
 
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