Transom Angle and effect in Outboard Trim

[Andrew Leigh]

Many thanks. I have peace of mind and am not about to embark on the curve of diminishing returns. Lots of expensive changes for minimal improvement. The boat after all is primarily a small recreational fishing vessel and not a performance craft.

I learnt a lot during this exercise, knowledge gained is never wasted.

I think I will explore the props though. I have been looking to get SS props again in any event, so this was already on the cards.

Happy New Year to you as well.

 

[Cajunpockettunnel]

Many thanks. I have peace of mind and am not about to embark on the curve of diminishing returns. Lots of expensive changes for minimal improvement. The boat after all is primarily a small recreational fishing vessel and not a performance craft.

I learnt a lot during this exercise, knowledge gained is never wasted.

I think I will explore the props though. I have been looking to get SS props again in any event, so this was already on the cards.

Happy New Year to you as well.

That is where I would start. The propellors. My buddy Fast Eddy has been in the prop business forever. He can make a pad riding boat go really fast with his knowledge. That is why I suggested it. Good luck and Happy New Year!

 

[Andrew Leigh]

That is where I would start. The propellors. My buddy Fast Eddy has been in the prop business forever. He can make a pad riding boat go really fast with his knowledge. That is why I suggested it. Good luck and Happy New Year!

Many thanks and a Happy New Year to you to. Quite a knowledge swap triangle, the U.S / Sweden and South Africa. Man, the internet can be used for such good stuff.

 

[Cajunpockettunnel]

Many thanks and a Happy New Year to you to. Quite a knowledge swap triangle, the U.S / Sweden and South Africa. Man, the internet can be used for such good stuff.

Indeed my friend. I believe in keeping things simple. A boat will only go as fast as it will go, period. My heavy 16 footer runs right at 40 with a 75.

 

[Andrew Leigh]

Morning all,

Some feedback, I took the boat out yesterday, along with my brother to assist in the measurement. The chop was worse than expected, and in short order my brother got seasick which rendered measurement impossible. I certainly was not leaving the Helm when at full tilt on those seas. The only reliable measurement I got was 27.7 knots at WOT, my recollect was 27.0 knots so we are still in the same ballpark.

We are going to take the boat to inland waters in order to get flat water to enable accurate measurements. I do not think that where I stay I will ever get waters that are conducive for testing, other than boat handling that is. When, I am unsure, but I would like to get these numbers as it will also assist in determining the improvement of the new props when I get them.

I will post the results when I get them, if @baeckmo still feels inclined, it would be nice to get the results of the computations.

 

[Cajunpockettunnel]

Morning all,

Some feedback, I took the boat out yesterday, along with my brother to assist in the measurement. The chop was worse than expected, and in short order my brother got seasick which rendered measurement impossible. I certainly was not leaving the Helm when at full tilt on those seas. The only reliable measurement I got was 27.7 knots at WOT, my recollect was 27.0 knots so we are still in the same ballpark.

We are going to take the boat to inland waters in order to get flat water to enable accurate measurements. I do not think that where I stay I will ever get waters that are conducive for testing, other than boat handling that is. When, I am unsure, but I would like to get these numbers as it will also assist in determining the improvement of the new props when I get them.

I will post the results when I get them, if @baeckmo still feels inclined, it would be nice to get the results of the computations.

If you dont want to break the bank on props, look into Solas. They are not too expensive.

 

[Andrew Leigh]

As it so happens, I have 2 x Solas New Saturn 3 Blade props sitting in my Shopping basket at Amazon . But they do not ship to South Africa. There are intermediaries that do, like MyUS in Florida that I have used in the past but it just gets so expensive. Double shipping, double handling, commission to handle the goods, so busy checking the math (dunno why, it is what it is). Fortunately, our local currency has gained 14% against the mighty U.S $ so probably still cheap at the price. I have not found a 11 1/8 x 13 performance prop, not sure there are any, all my reading is that aluminum props have very little cupping while standard stainless-steel props are naturally better cupped. Perhaps Fast Eddy could chime in, would you care to ask him?

 

[Cajunpockettunnel]

As it so happens, I have 2 x Solas New Saturn 3 Blade props sitting in my Shopping basket at Amazon . But they do not ship to South Africa. There are intermediaries that do, like MyUS in Florida that I have used in the past but it just gets so expensive. Double shipping, double handling, commission to handle the goods, so busy checking the math (dunno why, it is what it is). Fortunately, our local currency has gained 14% against the mighty U.S $ so probably still cheap at the price. I have not found a 11 1/8 x 13 performance prop, not sure there are any, all my reading is that aluminum props have very little cupping while standard stainless-steel props are naturally better cupped. Perhaps Fast Eddy could chime in, would you care to ask him?

Fast Eddy is not on this site. He is old school, knocking on 70 years old I believe. Give me the year of your engines, make,and model if you can. (I know they are Yamahas) but, I can call him tomorrow with your info and see what the shipping would be. And, if he has an email address I can put you in contact with him and y'all can discuss it there. Sounds good?

 

[Darkzillicon]

I may have missed some of the other comments, but typically two 40 hp are putting 80 hp out total and at a much greater weight than a single 90 hp. small boats are notoriously difficult to trim and much of the early advantages that you get from a fiberglass hull is lost later to drag on the rib tubes. It’s probably not what you want to hear, but I would say you may want to start with seeing if you can swap out the outboard first it will probably solve all your problems as others have mentioned a 15° transom is pretty standard on a small rib

 

[Cajunpockettunnel]

I may have missed some of the other comments, but typically two 40 hp are putting 80 hp out total and at a much greater weight than a single 90 hp. small boats are notoriously difficult to trim and much of the early advantages that you get from a fiberglass hull is lost later to drag on the rib tubes. It’s probably not what you want to hear, but I would say you may want to start with seeing if you can swap out the outboard first it will probably solve all your problems as others have mentioned a 15° transom is pretty standard on a small rib

Not everybody can afford that buddy. Aluminum props do have lots of slip. Start simple.

 

[philSweet]

@Darkzillicon Apparently South Africa requires twins to go any distance off shore. They don't require a suitable boat, but they do require twins The US has no restrictions and a very busy private towboat industry. Of course in the US, the gulf stream doesn't send you to Mozambique, just South Carolina.

 

[baeckmo]

Aluminum props do have lots of slip.

Naah, that is not true. The difference between Al and SS is that the SS blades can be made thinner for a specific blade load, which normally (but not always) is a benefit for a high speed prop, where there is substantial amounts of cavitation and/or ventilation. For the speed range in this case, a good Al screw is just fine. A reasonably competent propsmith can easily add some cup and rake to the existing screws.

 

[fallguy]

Michigan Wheel makes some cheap al props. But far better to find a good prop shop and get yours tweaked to all the recommendations offered.

I still think the fact you go to near stall trimmed down means your starting position is very negative. Why don’t you get some washers and loosen the engine and add them to the bottom. It would be a cheap way to test things. I’d not run that way all the time. For a 7” mounting, you need about 0.368” or 9.3mm for 3 degrees of change. That could be plan B after prop changes.

 

[Andrew Leigh]

Yamaha 40XWT bought in 2006. I think the model stayed the same from the mid 90's. It is a European model number so he may battle. It is the standard 2 cylinder version of the 40hp with trim and tilt and elecric and manual start.

 

[Andrew Leigh]

@Darkzillicon Apparently South Africa requires twins to go any distance off shore. They don't require a suitable boat, but they do require twins The US has no restrictions and a very busy private towboat industry. Of course in the US, the gulf stream doesn't send you to Mozambique, just South Carolina.

Hijack on.

I know that comment was tongue in cheek but we have many rules.
I have a CAT C registered boat and may venture no more than 10Nm from the coastline. To do this I need, on board;
An annual Certificate of Fitness which is signed off post an annual inspection. For ocean going vessels a minimum of 60% fitted buoyancy, inland 30%.
A Skippers License for my class boat.
A Radio Operators License and suitable VHF radio. We are required to use the correct radio protocols.
In a sealed floatable emergency container; Emergency medical kit, two types of flares and one smoke, basic tool kit, space blankets, pontoon repair kit, signaling mirror and manual horn to sound an alarm, red plastic sheet for air visibility when in distress
A fire extinguisher per motor
Oars / paddles
+20% reserve fuel for the planned voyage
Bottled water
Independent batteries and battery circuits
Manual steering mechanism
Capsize rope.
Anchor with sufficient chain.
Approved life jacket per crew.

In the absence of private towing companies, and scattered NSRI stations, I am not unhappy with the above requirements.

In another solid hull boat I owned, I struck a submerged log which ripped a large hole in my hull. Without that 30% buoyancy I would have lost the boat. I had 3 grandbuddies on board as well as my son and DIL. Managed to limp to an island in the middle of the reservoir. Friend came and took the kids and then towed my son and me in. Grandkids thought it was a wonderful adventure .

Hijack off.