Logo
FULL SITE MENU

470 owners, what prop size?

P

pugetsounder

Advanced Contributor
Having problems with RPMs and prop size. What prop sizes are you running. What drive ratio do you have, RPM range are you cruising and what speeds are you getting. Also what size boat do you guys have. I have a 14" x 21P but my RPMs vs speed are way off. Anybody running a bigger prop? Thanks.
 
pugetsounder said:
Having problems with RPMs and prop size. What prop sizes are you running. What drive ratio do you have, RPM range are you cruising and what speeds are you getting. Also what size boat do you guys have. I have a 14" x 21P but my RPMs vs speed are way off. Anybody running a bigger prop? Thanks.
Click to expand...

my 470 mcm with the 20ft wellcraft 3000lbs has the 14 X19 alum. prop. saltwater I can get almost 4000rpm. fresh water 4200rpm ..

with a boat wt of 2400-3200lbs 20-25ft you need a 15 X 19.

most of the older props used like the one on my boat are no longer made seen a 14.5X19 prop available diameter don't matter too much ..14 to 15.5 is good ..

at WOT what are you doing for RPM ?
 
"at WOT what are you doing for RPM ?"

I can push it to 5100 that's why I'm perplexed.

I'm on my second outdrive so I'm sure it's a 1.84:1 gearing just like the last one. The boat weighs 2600 loaded. I feel like a 15" x 23P might slow the engine down a bit but I would like to know what my RPMs will be before forking over the cash.

Wow only one other 470 owner?
 
pugetsounder said:
"at WOT what are you doing for RPM ?"

I can push it to 5100 that's why I'm perplexed.

I'm on my second outdrive so I'm sure it's a 1.84:1 gearing just like the last one. The boat weighs 2600 loaded. I feel like a 15" x 23P might slow the engine down a bit but I would like to know what my RPMs will be before forking over the cash.

Wow only one other 470 owner?
Click to expand...

I would put a known good tach on that motor to insure your RPM is correct , using the boat tach must be off as the spec for this engine is 4200RPM..

using the prop chart and with my experience you should be using the 21 pitch

what boat is this / yr manufacturer /length ?


drive is 165 or 184 ratio so the prop used does not change same 21pitch.

165 is used on smaller 19ft and less light weight boats..
 
Bt Doctur said:
Discussion Forum: 470 Talk
Click to expand...

Thanks doctor B I forget about that site sometimes. A member there named Island85 did a calculation for the 470 motor and 1.84 outdrive and he came up with a 13 3/8" X 25P size prop for up to 3000lbs. Gives me something to go on.

Prop sizing seems to be an art as well as a science. I tried a 14" X 19P on my boat last weekend and the performance was horrible. It got out of the hole quick enough but the RPM range was quite short. at 3400 I was falling off plane and at 4200 I was only reaching 24 MPH. So now I need to figure out if I need a bigger prop or more pitch depending on performance like hole shot vs top speed. I may try a 14.5" X 23P as well and see what happens. Unfortunately the closest prop shop that will do loaners is 30 miles away. Due to high rents in Seattle boating supply stores are getting in short supply. I will keep my ski prop for the lake but cruising in the islands it's nice to keep the RPMs down while maintaining a good speed on calm water.
 
j cat said:
I would put a known good tach on that motor to insure your RPM is correct , using the boat tach must be off as the spec for this engine is 4200RPM..

using the prop chart and with my experience you should be using the 21 pitch

what boat is this / yr manufacturer /length ?


drive is 165 or 184 ratio so the prop used does not change same 21pitch.

165 is used on smaller 19ft and less light weight boats..
Click to expand...

j_cat I put a dwell/tach on and the boat tach is within 100 RPMS. I know the outdrive was not original to the boat when I bought it so the prop probably came with the drive. I changed drives just to see if the drive on the boat wasn't a 1.98 that got installed by mistake but it wasn't. That turned out to be a huge waste of money and time but everything in the drive is good now. I thought about a 1.65 drive but they are in short supply used. Unless you have one built by installing gears but I will leave that to the pros with the proper tools. My drive is in great shape as is so I will keep it and save the money.
 
pugetsounder said:
j_cat I put a dwell/tach on and the boat tach is within 100 RPMS. I know the outdrive was not original to the boat when I bought it so the prop probably came with the drive. I changed drives just to see if the drive on the boat wasn't a 1.98 that got installed by mistake but it wasn't. That turned out to be a huge waste of money and time but everything in the drive is good now. I thought about a 1.65 drive but they are in short supply used. Unless you have one built by installing gears but I will leave that to the pros with the proper tools. My drive is in great shape as is so I will keep it and save the money.
Click to expand...


so how did you get 5100rpm out of the 470 ?

thats a good way to blow the engine .. 4200rpm max...

the 165 ratio is a optional drive ratio not that common..470 did not come with the 1.98 ratio but I guess if its used its possible...


maybe the prop is slipping ?
 
j cat said:
how did you resolve the high RPM at wot..

bad prop?
Click to expand...

Not yet haven't had time. I'm hoping to borrow a 14" x 23P prop and see how much a difference it makes. Or I can just throw down a hundred and cross my fingers. A guy at the propr shop in Tacoma said that for every step in pitch there will be 350-400 rpm change. Does that sound about right? If so then 23P should be right on the money by lowering 700 - 800 rpms, from 5100 down to 4300 or so. Enough to make me feel better about not blowing my engine and being able to keep up with friends with smaller motors.
 
pugetsounder said:
Not yet haven't had time. I'm hoping to borrow a 14" x 23P prop and see how much a difference it makes. Or I can just throw down a hundred and cross my fingers. A guy at the propr shop in Tacoma said that for every step in pitch there will be 350-400 rpm change. Does that sound about right? If so then 23P should be right on the money by lowering 700 - 800 rpms, from 5100 down to 4300 or so. Enough to make me feel better about not blowing my engine and being able to keep up with friends with smaller motors.
Click to expand...


the prop guy is correct ..
 
I bought a new prop today 14" x 23P can't wait to try it out this weekend. That propr guy said I would only see a 500rpm drop max. We'll see, it only cost me $100 to find out :confused: I guess if my WOT rpms are closer to 4500 it's still way better than 5000.
 
Hey pugetsounder, take a look on youtube under "Mercruiser 470". There is a clip that is pretty impressive about the use of a 23 pitch prop on I think a classic glasstron 174. I watched the clip several times and could not believe it. The guy is saying that he is getting up to 52.2 miles per hour on gps. I watched the clip and by the looks of the water and buoy flying by I do not doubt that he is reaching close to 50 mph. However, the boat that he is running has the four barrel 190 hp (I think)
I have a 176 hull and was pondering what prop to run on mine also. It is a trial and error type of thing. It really depends on power to weight ratio and the type and condition of your hull. I was asking the same question to a boat mechanic of 40 years and he told me that you must still be able to reach wot. If you cannot attain wot then you are overpropped. By the sound of what rpms you are pushing I would definitely think that you are way underpropped or the hub on the prop is shot. Take the time and enter the youtube site if you can. I think the authors name is rattan?
P.S. I think that thing is going to fly with the 23. I am curious as to how you make out so as to have an idea how to prop my boat as well! lol only the best, Tom
 
I thought wot was 4200-4600 on that motor.

Speed does not matter in relation to wot. It is what we want but not what we need.

The prop must allow you to achieve wot, in relation to what the maker recommended.

Say with trial and error a SS steel prop will allow you to get 4200-4600 rpm wot.

You may be going 50 + .

Aluminum will lets say get you to 4200-4600 but only 40, of course your going to take the SS if, you are will to pay the price. Big difference in price.

Me 20 maybe 25 mph is good enough for me i only go to wot on the way in once or twice a season, and for no more then 2 minutes to compare to past runs.

I run 1/2 - 3/4 throttle anyway. Aluminum cause i believe what they say about SS props being gear eaters.
 
Well I took the boat out this weekend under gray and rainy skies. Some summer here in the PNW!

So at WOT I am down to 4600 RPMs and top speed per GPS of 36 MPH. Cruising speed at 3600 I am around 27 -28 which is in improvement of 2-3 mph but still think the boat could do better. The down side is it takes longer to get out of the hole and on plane but not by much. I'm not trying to break a speed record with this thing but I would like to cruise at 30+ at 3600 RPMs where I normally cruise at. Top speed for this boat should be around 40. So I dunno. At least I'm less worried about blowing the motor due to over revving. I saw a 13 3/8" x 25P for $57 online I may try for kicks.

Chief I udually cruise at 2/3 to 3/4 throttle but using WOT terms to find the right prop. I still have a hard time believing the WOT RPMs on the 470 are only 4200 but that's what the tag says.

Browning I tried 2 seperate props with different style hubs so I think the hub is fine. 52 MPH? If it's a light boat and a racing prop I could see the possibility.
 
pugetsounder said:
Well I took the boat out this weekend under gray and rainy skies. Some summer here in the PNW!

So at WOT I am down to 4600 RPMs and top speed per GPS of 36 MPH. Cruising speed at 3600 I am around 27 -28 which is in improvement of 2-3 mph but still think the boat could do better. The down side is it takes longer to get out of the hole and on plane but not by much. I'm not trying to break a speed record with this thing but I would like to cruise at 30+ at 3600 RPMs where I normally cruise at. Top speed for this boat should be around 40. So I dunno. At least I'm less worried about blowing the motor due to over revving. I saw a 13 3/8" x 25P for $57 online I may try for kicks.

Chief I udually cruise at 2/3 to 3/4 throttle but using WOT terms to find the right prop. I still have a hard time believing the WOT RPMs on the 470 are only 4200 but that's what the tag says.

Browning I tried 2 seperate props with different style hubs so I think the hub is fine. 52 MPH? If it's a light boat and a racing prop I could see the possibility.
Click to expand...

what is the width and hull type ?
 
Last edited:
LOA 19', Beam is 78", hull type is deep V, clean and waxed. I'm guessing max gross weight fully loaded is around 3200 but most times the boat is around 2600. 2200 dry.

I don't know if the pertronix has something to do with it. I know I gave a kit to my father-in-law to put on his 1973 120HP and he said his RPMs got quite a bit higher than before with the points and condensor. Never heard of anybody having to buy a different prop due to the Pertronix kit.
 
pugetsounder said:
LOA 19', Beam is 78", hull type is deep V, clean and waxed. I'm guessing max gross weight fully loaded is around 3200 but most times the boat is around 2600. 2200 dry.

I don't know if the pertronix has something to do with it. I know I gave a kit to my father-in-law to put on his 1973 120HP and he said his RPMs got quite a bit higher than before with the points and condensor. Never heard of anybody having to buy a different prop due to the Pertronix kit.
Click to expand...

thats interesting about how this changed with the pertronix ignitor kit. I noticed not an increase in rpm at WOT , but for sure more power , immediate power when the throttle was increased. the engine never ran this good since 1987. My 20ft wellcraft is 3200lbs deep vee 50 gallon tank . I do WOT at 4200rpm now with the 14.5 X 19 prop.
 
Sorry, Pugetsounder and jcat for not responding. I was down the Jersey shore on vacation for the past week. I am almost positive that the wot for my 1988 180 hp is 4600-4800 rpm. I read it on the valve cover I think. The two barrels spin a little slower and I think it to be 4200-4600 if all is well. I also agree with you chiefallen about not needing to travel that fast. It would have to be flat as glass and when do you see it like that? It just startled me a little about how much speed he was able to achieve from the 23. I guess the boat must be pretty light also. Not to change the subject but I had some luck in Toms River N.J. this past week. I found a fiberglass engine cowling at a junk yard off of a Wellcraft that was specifically designed for the 470 engine. (24"high x 31" wide) I have been looking for 2 years with no luck until now.(like hitting the lottery) It has rod holders,cup holders,hinges,teak, etc. It needs a good cleaning up and a little paint and hopefully I should be good to go. Thank God! lol I am glad you are getting the rpms down on your boat Pugetsounder and agree that you should run around 2/3 or 3/4 (so say 3600-3800 rpm). Talk to you guys later, Tom
 
Last edited:
Old post but I just bought a 13 3/8" X 25P prop from iboat. $70 delivered. I couldn't pass it up. Now hopefully the wetaher cooperates and I can test it out this weekend and see if I can lower my RPMs and raise my WOT speed a bit.
 
Back
Top