1984 evenrude 70hp prop

[dbmmorin]

I am looking to try a 4 blade prop on my 1984 evenrude 70. Its on a 1963 starcraft cheiftain. The bow rides high and I want to get it down some. I read a 4 blade prop will help with that. I know nothing about props and pitch and that stuff. Will a 4 blade prop be the same size in circumference as a three blade? Really kind of clueless here it looks like it can be from and 11 pitch to a 17 pitch.

Thanks
Mike

 

[flyingscott]

Trim the motor down or move the pin

 

[Texasmark]

You are talking about a 22' boat in the Chieftain and 70 ponies aren't enough to do you much good. I'll go with your current prop pitch to be closer to the 11" you posted rather than the 17. I ran a 17-19 on a 15' Kingfisher bass boat with a 70 rude. Light boat, maybe 600# and just me. Your problem is not enough power to get out of the hole and up on top, especially if you have ANYTHING else in the boat besides you! Doing what scott said may help but don't hold your breath. even if the lower unit is up against the transom!

 

[dbmmorin]

So I have some more information. The current prop is a 15 pitch. The pin is all the way down and the boat runs about 27 - 26mph with 2 passengers and is on plane at full speed. If I could cruise at 18 to 20 mph on plane I would be happy for now. It is not in the budget for new power until next year. I am looking to put a 115 to 125 hp two stroke on there. I don't think a 90 would make that much difference. An Evenrude ETEC preferably. I need an new prop. the other one got a couple good dings in it. Has a little vibration. Make a good spare in a jam. My thought is to go ti a 4 blade 13 pitch and see what happens. I am not sire of the speed loss, but if the top speed drops a little but it cruises more on plan that will make me happy for now. Do you think it will help going to a 4 blade and help keep the bow down? Do I have to drop 2 pitches or can I just drop 1? The Solas 13.5 4 blade in a 13 pitch is around 120 so not an expensive venture. Worth a try?

 

[WesNewell]

Hull shape determines plane speed. I don't think changing props will change that much. Only help get out of the hole faster. If you want to plane at a slower speed, trim tabs may help, or lose some weight.

 

[dbmmorin]

OK, thanks. I did bring a salad to work today

 

[ahicks]

Buy a tach! It's the ONLY way to make informed decisions when it comes to prop choice. If we knew how many rpm that current 15" prop was turning at wide open throttle, we might be able to offer some informed advice.

All else being equal, I think a 4 blade prop WILL hold a boat on plane to a lower speed than a 3 blade.

 

[Texasmark]

Impressive, although having had an 18' Holiday I/O that hull is light and easily planed. Agree with hicks on the tachometer.

You/we have to have a starting point. Get a tach, even if it's a "Tiny Tach", install it, get back to your 26-7 MPH and get an RPM number. Come back with that and we'll shoot for an aluminum 4 blade that would probably work for you! iboats sells props and for that engine you'd probably be looking at a hundred bucks, tax,title, and license.

 

[racerone]

So how big is the boat in question / post #1-----And is the motor in good condition.----Changing a prop just to get the " happy RPM " is not the correct way to go about it.

 

[dbmmorin]

ok, ill order a tach today

 

[Texasmark]

So how big is the boat in question / post #1-----And is the motor in good condition.----Changing a prop just to get the " happy RPM " is not the correct way to go about it.

If he is getting 26 MPH out of a 22' aluminum cabin equipped boat with 2 additional passengers with a pre'85 OMC 70 hp, I'd say things are humming right along.

How's this for a set of numbers; data available with online google search. Go-fast supplied the optimum slip calculation used for a boat of the type used in this posting, generally speaking:

https://www.go-fast.com/Knowledge-ba...lip-calculator

That pretty much says that he is dead-nuts on with his existing prop pitch. Being in the center of the recommended RPM range of 4500-5500...the place where engine mfgrs. spec their HP (opinion) gives latitude for errors in the calculations of slip and rpms.

Bass boats want their bow high to get on the "Pad" thus getting the 65 mph speeds with minimum hull in the water. They want a bow high prop. What we have here is a slow speed boat that is underpowered so it needs some help which, as surmised, needs a stern lifting prop, like as surmised one would get with a 4 blade. Since he is in the lowest hole and doesn't have PTT to be a factor, can't improve on that.

I think I would get a new prop if the current is "dinged" up for starters, or if you just want to go to 4 blades have at it. Rule of thumb is drop 1" of pitch when going to 4 blades with the similarly designed 3 blade prop otherwise....primary concern is Rake differences in prop types; less for general purpose usage or high for high speed.... with your staying with the general purpose props of which I feel you are currently equipped. I've used Turning Point aluminum from iboats and they are good props with a great price, a premium aluminum.

After the prop change if not satisfied, then add trim tabs to the transom. Tabs WILL force the bow down, or the Whale's Tail type device for mounting on the lower unit of the outboard seems to be popular....I never needed one, nor wanted to put such on any of my engines over the years, but they apparently work, are cost effective, and seem to be popular.

My 2c and worth every penny you paid for it. Grin

 

[jimmbo]

I would say 26mph with a 70 hp on a 21 - 22 ft boat is doing very well. Hopefully the engine is running near the 5400 - 5500 rpm mark.

OMC Prop charts say for a 18 - 22 ft boat, a 3 blade 13" and speed range of 15 - 26 mph...

Same chart shows the 15" for 16 - 19 ft boats with a speed range of 22 - 32mphj.

Based on the speed ranges(corresponding to RPM range at WOT, taking some Slippage into account), I would say you are running too much Pitch

However a Tach is needed to verify what RPM the engine is attaining


18 mph is about the speed where most boats are not on plane, and therefore B\ow High. The only way to bring the bow down to simulate Planing, and still keep your top speed is to add Controllable Afterplanes(trim Tabs). These will provide a Hook when needed but get out of the way for Top speed. More efficient than trimming the engine in, as the Prop thrust can still be horizontal