Propeller size
 

Updated information 8/3/16:

Hello, I have a 1970 20' Sea craft c.c.. It has a aluminum t-top, electronics box, original console with a 3/8" windshield extended to 6', 43 gal. Gas tank usually run 20 gal. Of gas, 20 gal. Live well, 2 batteries, 2 adults, I usually fish 8 months out of year. Boat is used in New York : jamacia bay, New York bight area. I rebuilt the transom for a 25"shaft four stroke engine 150 h.p. Yamaha, I would like to know if anyone can recommend a propeller size/pitch to get the boat to do 40 - 45 m.p.h. If not with 150h.p. Then perhaps with the 175h.p. Yamaha..

Greetings and welcome aboard.

Probably no answers cuz 150 4-stroke is not a good idea in my mind. Too much weight.460 to 490 lbs.

140 Suzuki or 115 2,1L mercury can provide low 40s at 410 and 360 lbs. I'd seriously reconsider your power decision.

Guys running older 150 2 strokes in the 400 lbs class w 2:1 gear ratios are swinging 14.5x17 or 15x15 stern lifting props. Probably swing more pitch with the 4 strokes due to gear ratios designed to overcome extra weight and alleged inefficiency.

Again, reconsider - 150 4 strokes are about 160 to 190 lbs more than what the boat was designed for. Speed gained is not worth the performance loss. The 20 is a heck of a little boat, when properly rigged.

Thanks McGillicuddy for your advice. I'm gonna repost with more info.

The main thing you need to do is get your RPM's in the right range. You mainly prop for rpm, stability and running conditions. A heavy motor like that you will want something cupped to lift the back of the boat to counter the weight (stern lifting prop). Far as MPH you just get what you get once you get the rest of the boat happy and your rpms in the right range. You can look for features to give you more mph but its a give and take. You may gain mph but loose ride quality or low end. I like to go fast and run larger hp motors than necessary sometimes but its a give and take when trying different props. I can run faster in the calm with one prop but it may not plane out well at low speeds on a choppy snotty day making the ride bad. It all depends on many factors like weight in the boat, balance of the weight, how loaded you run the boat during different trips, conditions of the water where you run the boat, what you are willing to live with or looking for in ride quality and even things like bottom paint creating drag can vary prop selection. If you are just trying to get more speed out of your hull and willing to give on the other areas you can get a smaller diameter prop with thinner blades. You can get the lower unit up higher and out of the water to loose drag. You can try semi surfacing props but make sure what ever you do you keep it in the right rpm range. You can deal with bad handling and ride to gain a few mph but your motor will not deal with being lugged with a over pitched prop dragging it down. Most look for the best ride and over all performance threw out the full range while keeping the motor in the right rpm range. If you get all that right you should have a fair mph with it but at the same time you can do things to gain 4-5 mph and have a boat that rides like crap. There are so many different props out there and each will change the ride of the boat. Even two of the same type props like mercury mirage props. The newer model year A45 is a heavy thicker bladed where the older A40s are lighter and have thinner blades. I have seen a 3 mph difference between the two even tho they are the same prop size and pitch. Over the years the newer ones have gotten heaver and the slightest design changes effect the boat differently.

Rich 10, I moved your new post here to your original one since it was basically the same question with more information added to it. Trying to keep from having multiple threads with the same question filling the form. Plus you will get better responses keeping all the info in one thread so people can gauge what has been said and add to it.

I will add this and maybe it will help you out a little bit more on selecting a prop.

Im not really sure what you are asking but I am thinking you want prop suggestions for an Yamaha 150 or 175 to make sure you can get 40-45 mph out of it? Just odd due most people prop their boat looking at other conditions and speed is usually the last thing people worry about except for efficiency. Between different props you may see a 5-7 mph difference but not as much as you would think.

Far as pitch, you will pick this based on the rpm range on your outboard. You want the rpms at wide open throttle with a normal load on the boat to rev to the max recommended rpms of the outboard at least. If not slightly over the max recommended rpm this way if your loaded down more you will not lug the motor. I don't know what people run on their 4 stroke Yamahas but I would guess start with a 17p or 19p and go up or down from there to match your rpms to the prop pitch. Some one with a Yamaha can get you close pitch recommendation for you to start with. Far as diameter I would stay small if you are concerned about mph and getting in the 40-45 mph range. Some where around 14 1/2" or so. The larger 15" plus props will handle and bite the water better but slow you down speed wise. I would try a old model mercury vengeance prop, not a expensive prop but a good smaller dia prop that will give you a base line to start from. It will fit your Yamaha. The Yamaha salt water II props are good as well. You can buy or borrow one to try it out first and tune from there. The mercury fury or any of the newer props made from the x7 alloy can be faster than the standard ss models due to their lighter thinner blades. The new alloy makes them strong enough to lighten them up. If your man concern is about is getting the mph up you can work on dialing in your motor height. Raise the outboard as high as possible with out slipping the prop and get the av plate slightly out of the water. This will reduce drag and add more mph than a prop change most times.

It may be best if you ask or research the form and look at hulls like yours with the Yamaha 150 - 175s and see what they were running far as props and what there posted performance numbers were. You may can get a good idea what to expect from this combination. That will probably help you the most picking an prop to go on yours.