Repowering pontoon from 40 hp to 90 hp CT where to start with prop.

[HelloThere]

I am repowering a 1995 Premier Sunsation 21' pontoon from a 40 hp 2 stroke to a Mercury 90 hp 4 stroke with the command thrust gear case. I asked a couple of dealers and they have suggested 10-13 pitch as a starting point. When I use Mercury's prop selector entering my boat weight as 1550lbs with 1000lbs of people and gear it gives me a range of 14-16 pitch with 14-16" diameter. Why such a big discrepancy? Is the dealer maybe not taking in account the CT gear ratio of 2.38? Is there somewhere that has a list of real life results that I can use? If I were to buy a prop today, I think I would go with a 4 blade 14x14 Merc Spitfire. Thoughts?

 

[WesNewell]

Don't know, but you definitely don't want anything less than a 13p, and a 14p would be ok with twice the weight.

 

[HelloThere]

The marine shop I went to had the 13 and 15 pitch spitfire. Going to give the 15 a go when I get my new motor installed next weekend. Hopefully it isn't too much prop.

 

[QBhoy]

Remember this engine only takes the larger prop models usually found on bigger V6 and upwards engines. Alpha too.

 

[ahicks]

I did something similar, moving from a 40hp Johnson to a 90 hp Honda on our 24' 'toon. There's so many variables the right prop is a roll of the dice really. I went with a 13p which worked out OK. Empty with just me on board, the boat would run 24-25mph and about 6100 rpm. I spotted a NOS 15" on ebay (cheap!) and bought that prop. That turned out to be the better prop. Motor slowed to 5500, and speed went up 1mph. Better than that though, is the fact the motor doesn't sound anywhere near as busy at 5500 as it did at 6k. So the bigger prop was a win-win.

Since then, I've tried the same move with the 40 and 50hp Honda's I mess with quite a bit. Same story. They're rated to turn 6k, but seem much happier at 5000-5500. This is on both pontoons and fishing boats.

 

[HelloThere]

Thanks, good to hear. I hope that it works out that I redline it with just me in the boat and hit the sweet spot with passengers.

Is there a list someone is keeping that I can report my numbers to when I get my results?

 

[QBhoy]

Hi
Not sure If you saw my earlier comments but I’m not sure there is a 14x14 spitfire for the big gearcase you have. Might be wrong.

 

[Texasmark]

Thanks, good to hear. I hope that it works out that I redline it with just me in the boat and hit the sweet spot with passengers.

Is there a list someone is keeping that I can report my numbers to when I get my results?

go-fast.com and Mercury have "prop slip calculators" which will help. You plug in your haves and solve for your what-ifs. Problem with them is that they aren't totally inclusive which means if you change make/style of props or if you have a large diameter change, the numbers won't be accurate.

Prop slip is one if the boxes and depends on a lot of things. I don't know what is the usual prop slip for a Toon. For light-medium boats with fairly flat transom areas and rated power (max power for which boat it BIA certified), 15% is a good starting point. If you stay in the same prop, changing pitch a bit should not change slip all that much.

 

[ahicks]

Thanks, good to hear. I hope that it works out that I redline it with just me in the boat and hit the sweet spot with passengers.

Is there a list someone is keeping that I can report my numbers to when I get my results?

There's a section of this forum dedicated to pontoons that you could post your data in. Might make it easier to find if somebody is looking further down the line.

 

[HelloThere]

Hi
Not sure If you saw my earlier comments but I’m not sure there is a 14x14 spitfire for the big gearcase you have. Might be wrong.

It is what the Mercury Prop selector suggested for my motor. https://www.amazon.ca/Mercury-Marine.../dp/B073ZKDJQ3

They apparently sold them at least at one time. It is probably less diameter than what you were expecting because it is a 4 blade, so the diameter is made up for with the extra blade. With that said, I checked the box and found out they sold me a 14ish x 15 3 blade black max 3 that won't work.

 

[QBhoy]

It is what the Mercury Prop selector suggested for my motor. https://www.amazon.ca/Mercury-Marine.../dp/B073ZKDJQ3

They apparently sold them at least at one time. It is probably less diameter than what you were expecting because it is a 4 blade, so the diameter is made up for with the extra blade. With that said, I checked the box and found out they sold me a 14ish x 15 3 blade black max 3 that won't work.

Fair play. I’ll apologise, haha.
Think a few will be caught out by the need for the bigger props on these CT models.
How are you finding it ? I’m literally blown away with my 115CT. It’s incredible and I and many others are convinced it’s pitting out way more power than the badge suggests. It’s outperforming 18ft usual brand bowriders with the 3.0 mercruiser, even though I have a heavier hull and 20 less HP (apparently) an amazing thing. It even outperformed the 125 optimax in every aspect in a recent same boat test.

 

[HelloThere]

I'll say it is a big improvement over my 40 hp! I don't have anything really to compare it to, but it feels like it has really good power. I ended up with a Black Max 15.5x15 blade. I topped out at 24 mph with just me in the boat. I don't have a tachometer installed yet, so hard to tell if it is optimized, but it feels real close to where it should be.

 

[QBhoy]

I'll say it is a big improvement over my 40 hp! I don't have anything really to compare it to, but it feels like it has really good power. I ended up with a Black Max 15.5x15 blade. I topped out at 24 mph with just me in the boat. I don't have a tachometer installed yet, so hard to tell if it is optimized, but it feels real close to where it should be.

Great stuff. I’m loving mine. Watched a few video now of them doing really well against 2 strokes. So impressed. Get yourself a vessel view mobile for it. Great wee gadget if you don’t have the smart craft gauges.

 

[HelloThere]

Great stuff. I’m loving mine. Watched a few video now of them doing really well against 2 strokes. So impressed. Get yourself a vessel view mobile for it. Great wee gadget if you don’t have the smart craft gauges.

Tha's actually not a bad idea. What's another $300 at this point. It is either that or buy a $50 tach and mess around with install on old wires. What are the major pieces of information that would be able to track for my engine?

 

[QBhoy]

Tha's actually not a bad idea. What's another $300 at this point. It is either that or buy a $50 tach and mess around with install on old wires. What are the major pieces of information that would be able to track for my engine?

Pretty much everything. There will be more than I can remember but here is what I can;

oil pressure
oil temp
water pressure
water temp
gps speed
average speed
trip journey
mpg
current fuel flow which is deadly accurate calculated straight from the ecu
fuel left
total fuel
range left
time left to run
best cruise range calculation
service countdown for each serviceable item
hours
time
local dealers
gps map with you being able to add references on it
battery voltage
exact rpm to the 10th. Deadly accurate and great for prop setting up
max speed
best holeshot time recordable
trim level

there are more but I’m running out of thumb prints haha.

 

[QBhoy]

Oh and of course any faults and diagnostics etc. If you log in, it will actually tell you the actual fault. Not just a code.
Steering angle (dependant on your set up)
depth (if you have a suitable sounder to talk to it)

 

[HelloThere]

Pretty much everything. There will be more than I can remember but here is what I can;

oil pressure
oil temp
water pressure
water temp
gps speed
average speed
trip journey
mpg
current fuel flow which is deadly accurate calculated straight from the ecu
fuel left
total fuel
range left
time left to run
best cruise range calculation
service countdown for each serviceable item
hours
time
local dealers
gps map with you being able to add references on it
battery voltage
exact rpm to the 10th. Deadly accurate and great for prop setting up
max speed
best holeshot time recordable
trim level

there are more but I’m running out of thumb prints haha.

Lol, that is quite the list. I'm sold! Thanks for the suggestion!