Small 4Hp Suzuki prop choice for heavy dinghy?

[Franziska]

Hi,

we have an about 20 year old 2 stroke 4hp Suzuki engine.
Model DT4 from 2000.
It's previous owner hardly used it and only in fresh water.
Its virtually new.

Starts very easily, runs fine, but, from about half throttle to full it feels choked.

To me it feels that the small prop has not enough grip in the water to push our heavy 3.2m dinghy. So I suspect bigger blade area and different pitch might help.

The current props says 7 1/2 x 6 1/2 on it.
Prop model seems C610.

I had a lengthy thread on this about two years ago, but we never found a solution.

Now I have someone who is offering me a choice for a different prop. Here is the proposal:

"... These are the Suzuki original propellers abailable in Barcelona with their pitch and price:    

58110-91JL0-019 HELICE* 3 X 7-1/2 X 6 : 128,65€

58110-91JM0-019 HELICE* 3 X7-1/2 X 6-1/2 : 128,70€

58110-91JN0-019 HELICE* 3 X 7-1/2 X 7 : 128,70€


The non-original propellers are:

helice "amita 3" 7.9x6.75: 130,96€

helice aluminio 7,90*6,75 derecha: 59,55... "

I was wondering if someone with real world experience on props can suggest what to chose.
I learned about prop choice many years ago, but never had to dig into this in the last 20 years, so I appreciate help.

Thanks,

Franziska
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[krid2000]

The prices seem high for a simple small prop.
You can buy small Suzuki compatible props on amazon for $35.
Why not try out one or two of the sizes you mention for cheap and see how they work out?
Worst case, you have one or two spare prop.

Dirk

[Rumrace]

You might consider a 4 blade prop
Yamaha has a gear and prop high torque option for 9.9 15, and 20. They never offered it on two strokes.
Two strokes are not know for torque needed to push a pontoon or sailboat.
I have a 4hp Yamaha 4 stroke I’ve moved a 6 ton SeaRay by strapping the Dinghy to the side of the Ray.

You may have resin build up in fuel pump or carb jets. Captain Phab Carb and injector spray is an instant winter resin fuel preserver remover. The Suzuki fuel pumps I’ve seen can swell and corrode just from sitting. Suspect anything rubber especially around fuel.

[Sailmonkey]

Our 5hp 2stroke was struggling to move our dinghy with two people it.

I changed the original prop (8” pitch I think) to a solas 7”pitch prop of the same diameter. What a difference!!

So if I was in your shoes I’d drop an inch or 1-1/2” off of you pitch.

[Rumrace]

Thank you SAilmonkey Great Advise.
A prop motor and hull speed have some complex maths going on.
Dropping the pitch but increasing the blade area helps an underpowered motor move along. Increase the pitch you’ll increase the dreaded prop. If the prop is a 7 1/2 pitch and it moves the boat across the water at cruise speed 7 1/2” that is the sweet spot.

I had counter rotating 19” 21p stainless props on a boat 19” 20p would have allowed the motors past the 4400 rpm redline. 22p would never achieve top speed labour on to plane and waste fuel.

[Franziska]

Thanks guys.

So, out of the prop choice in my opening post I should go for the

3 X 7-1/2 X 6

Which would have 1/2" less pitch than the original 7 1/2 x 6 1/2 or alternatively to the

3" 7.9x6.75

which seems to have a larger diameter, but also a slightly larger pitch than the current one.

Alternatively I shall try to find a

3" 7 1/2 x 5.5

Or should I pick something else?

The engine is a Suzuki DT4 from 2000.

Haven't seen 4 bladed props for such a small outboard.

[GreenWave]

Unfortunately I cannot weigh in on the prop selection - just no real experience in this scenario. However, what you are experiencing is the reality that your motor is bogged down due to too much weight to push - most likely too much in the back of the dinghy. Moving weight forward will help some. Putting a Dol-phin foil on your outboard (google them) will help some as well. How much does your dinghy weigh? And does it have a double floor?

[Franziska]

Originally Posted by GreenWave

Unfortunately I cannot weigh in on the prop selection - just no real experience in this scenario. However, what you are experiencing is the reality that your motor is bogged down due to too much weight to push - most likely too much in the back of the dinghy. Moving weight forward will help some. Putting a Dol-phin foil on your outboard (google them) will help some as well. How much does your dinghy weigh? And does it have a double floor?

It's about 45kg and is fully decked.

Basically like a Laser sailing dinghy but with a shorter hull, more freeboard and the possibility to row, sail and motor it.

We also have a big forward hatch / trunk in it to store any stuff/luggage we like to get to the mothership dry.

Works nicely, apart from the chocked engine performance.

Might try a 6hp but we feel the 20kg of the 4hp are already a pain.
So probably next try is to get a prop with lower pitch.

[Rumrace]

No one can answer that question accurately

Go to this web site and follow your motor boat weights etc.
The Altima is what most Pick. Composite is a real option also

https://www.solaspropellers.com/index.php/component/zoo/item/high-thrust-propeller?category_id=189

[jim King]

Check eBay for some low cost props. Some new some used but almost always less than you find at the chandlery. I found a three blade prop for my Tohatsu 3 hp for just $35.

[Franziska]

So, it's been a while, but I thought I provide some feedback.

We had the carburetor revised and changed the prop to a lower pitch one.

The choking is gone and the hard dinghy planes now with 7-8kn.
It never did that previously.

Very pleased with this.

I just update both old threads to help anyone having similar issue.

[Don C L]

Originally Posted by Franziska

So, it's been a while, but I thought I provide some feedback.

We had the carburetor revised and changed the prop to a lower pitch one.

The choking is gone and the hard dinghy planes now with 7-8kn.
It never did that previously.

Very pleased with this.

I just update both old threads to help anyone having similar issue.

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Thanks for the follow-up!