Seacraft 32. Twin 200 Yamaha 4 strokes!

[captbone]

Originally Posted by FishStretcher

The torque on the the gears will be the same, true. But the force from the higher thrust at lower speed will be resolved in the lower unit bearings. And the motor mounts. I don't design lower units, but I have a suspicion that this is something you can get away with for a time. Or at least get home and then tear down a LU and put new bearings in it. I suspect the gear teeth will be fine.

Just a guess.

The thrust is all the same. If the thrust was higher the boat would go faster. The speed in which the gearcase travels through the water or the amount of thrust producted at a given RPM is not relevant since the max thrust would be identical at WOT as long as it can reach the recommended wot rpm and peak hp.

Outboard engines that push barges, heavy boats, or tow a lot do not have a decrease in life simply because the engine can only product so much load, torque and thrust. Propped correctly, their is no increase in wear, tear or load regardless of what the outboard is pushing or the prop. Max is at WOT regardless of prop.

Just my 2 cents. I would gladly chip in to see the owner run a 13 pitch prop on one of the twins to see it break 30mph.

[gofastsandman]

Originally Posted by captbone

The thrust is all the same. If the thrust was higher the boat would go faster. The speed in which the gearcase travels through the water or the amount of thrust producted at a given RPM is not relevant since the max thrust would be identical at WOT as long as it can reach the recommended wot rpm and peak hp.

Outboard engines that push barges, heavy boats, or tow a lot do not have a decrease in life simply because the engine can only product so much load, torque and thrust. Propped correctly, their is no increase in wear, tear or load regardless of what the outboard is pushing or the prop. Max is at WOT regardless of prop.

Just my 2 cents. I would gladly chip in to see the owner run a 13 pitch prop on one of the twins to see it break 30mph.

Let me take my little brain off the short bus for a minnow. If 1 LU is being asked to push the same weight as 2 , is that not increasing the loads of torsion, shear, and compression?

[Bushwacker]

Originally Posted by gofastsandman

Let me take my little brain off the short bus for a minnow. If 1 LU is being asked to push the same weight as 2 , is that not increasing the loads of torsion, shear, and compression?

If 1 LU was pushing boat at same speed as 2 motors, then yes all loads would be up by 2X . . . but it can only develop the thrust of 1 motor, so loads are not any higher. The lower thrust of just one motor is why speed will be lower.

Regarding a spare prop for twins, assuming one was counter rotating . . . wouldn't it make sense to buy a Piranha plastic prop hub and then 2 sets of plastic blades, 1 std and one counter rotating set? (Assuming they even make counter rotating blades!)

[captbone]

Originally Posted by Bushwacker

If 1 LU was pushing boat at same speed as 2 motors, then yes all loads would be up by 2X . . . but it can only develop the thrust of 1 motor, so loads are not any higher. The lower thrust of just one motor is why speed will be lower.

Regarding a spare prop for twins, assuming one was counter rotating . . . wouldn't it make sense to buy a Piranha plastic prop hub and then 2 sets of plastic blades, 1 std and one counter rotating set? (Assuming they even make counter rotating blades!)

Exactly. An outboard motor can only product so much thrust/HP/torque regardless of the propeller that is on it. The size of the propeller does not matter in that sense. For load on the engine, as long as you are propped correctly to hit WOT RPM that engine will not know anything is different.

The problem is when people try and put too large of propeller on their boat or increase the weight of the vessel without switching propellers. I would never attempt to plane off a 32ft Seacraft with a single 200hp if it is wearing the propeller it normally does. If you did try, the engine would only reach 3000-3500 rpm at wot and would have a huge load/stress on the powerhead. With a correct smaller pitch propeller (11 or 13 pitch) the engine would rev freely to the correct WOT and would never know it is not running normally.

I am currently running a single 225hp on a 25ft flybridge Bertram with no issue and am very happy. I am actually looking to upgrade to either a 31 Bertram or 31 Ocean Master and I plan on putting a single 250hp outboard on it. If I can get a 20-22mph cruise at 30 mph WOT, I would be thrilled. It will have huge tabs and doel fins to help with stern lift. The reason for this is because I never go more than 10 miles from my dock inland on nice days and am in no rush. I want as big of a boat as possible with the least amount of maintance and work. Twins are double trouble in my book. People laugh but it all depends on how you use your boat.

There was a 32 Seacraft with one blown engine for sale recently that I almost jumped on.

[Was12]

Today I tried to put the boat on plane with one motor and it ran up on plane. The motor was turning up 5200 RPM's and running about 28 MPH. The motors currently have 14 1/2 inch by 17 pitch.

[gofastsandman]

Originally Posted by Bushwacker

If 1 LU was pushing boat at same speed as 2 motors, then yes all loads would be up by 2X . . . but it can only develop the thrust of 1 motor, so loads are not any higher. The lower thrust of just one motor is why speed will be lower.

Regarding a spare prop for twins, assuming one was counter rotating . . . wouldn't it make sense to buy a Piranha plastic prop hub and then 2 sets of plastic blades, 1 std and one counter rotating set? (Assuming they even make counter rotating blades!)

Hi Ho back on the bus I go.

Got some Cobia today... Stop by the shop tomorrow.
Better stick to what I`m good at and leave science to scientists.

[captbone]

Originally Posted by Was12

Today I tried to put the boat on plane with one motor and it ran up on plane. The motor was turning up 5200 RPM's and running about 28 MPH. The motors currently have 14 1/2 inch by 17 pitch.

Awesome. With a 15 pitch or 13 pitch, I am sure she will break 30mph. It's all about set up. I see many people claiming their 23 Seacraft only tops out in the low 30s.