What prop should I have?

[johnathan.hall]

I am trying to see if I have the right prop and if not which one do I need?

1. My boat acceleration/hull shot is slow but top end is good
2. Quicksilver, stainless steel 3 blade unsure of model
3. Diameter is 14 and pitch is 21
4. WOT RPM is 4000 and speed is about 50mph
5. I have an mercruiser 898 type 1. From 1981 (serial # outdrive=5902360, transom plate=5878040, engine=5896259) HP is 198
6. Boat is a 1981, 18 foot RSB Fiberglass, sportsman

 

[steelespike]

Re: What prop should I have?

I'm not real expert on the older units but my limited research indicates a 898 with a 305 has a 1.65 gear ratio.
Prop calculator says it can't do 50 at 4000 rpm. Is your speed by gps?
Sometimes the gear ratio is on the outdrive.

 

[johnathan.hall]

Re: What prop should I have?

The outdrive shows a "1.50R". I have attached some pics of the outdrive showing the data plate and also a close up of the info on the prop itself.

Also my speed was based off of a buddies boat who was running with me he was at 47-49mph and i was slowly passing him.

 

[steelespike]

Re: What prop should I have?

Using the 1.50 ratio at 50 mph your slip is about 6% ,10% wopuld be more typical.The only way to be sure of your speed is to verify it with a gps. Accurate speed plus the ratio and the pitch gives us 3 of the 4 required parameters to estimate how accurate your performance is.I do suspect both your speed and your tach.
It does appear You need a little less prop decreasing about 2" to a 19" prop should add about 400 rpm.
If your tach was actually at 4300 that would put you at about 4700. If your max rated rpm is 4800 that puts you there.
If your actually at 4000 you you would need a 17" prop to get you close to 4800. That seems really low for
a 200 hp 18ft boat. I think you can see we really need at the least accurate speed from a gps.
Some cells have a gps app.
Dropping 4" to reach rpm will really boost hole shot but kill your top end.

 

[johnathan.hall]

Re: What prop should I have?

Ok the next time out we will gps it...would going from a 3 blade prop to a 4 blade change any of your suggestions?

 

[steelespike]

Re: What prop should I have?

Generally speaking a 4 blade" should"
improve hole shot,ability to stay on plane at reduced throttle,handle batter better at speed and around the dock.
If your boat was at normal rpm its usually suggested to reduce pitch one inch when going to a 4 blade.
This would help counter the resistance of the extra blade.
The boat would usually lose a little top end with a 4 blade.

 

[johnathan.hall]

Re: What prop should I have?

ok so for my current situation what path to a new prop do you recommend I take?

 

[johnathan.hall]

Re: What prop should I have?

So was looking through my manual and it looks like I am right at spec...which is 3800-4200 WOT. Does this mean my only safe option to increase hole shot is through a 20" - 4 blade?

 

[steelespike]

Re: What prop should I have?

I can't say the a 20" 4 blade is the only way but The 4 blade is said to provide stern lift out of the hole and bow lift at speed.
Also may improve the ability to stay on plane at reduced throttle and may provide better control around the dock.
Are you staying with a ss prop?

 

[johnathan.hall]

Re: What prop should I have?

Just because of cost if I get a new prop it will be aluminum. I will have the boat on the water Sunday with the hydro foil on...we will see how it does with just that on.