flybyjohn

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I am currently scratch building a seamaster and was thinking of making it a twin. I was going to use asp25 engines (37.00 each) however I have not seen any twin seamasters with 25s. I have seen one with 36's and he said it jumped off the water and then some with 40's. Do you all think that 25's will big big enough. The plans I am using are the modified Ken Willards? plan. It said that it was reduced from 7 lbs to 5.5lbs. and said not to fly with more than a .40 size. One more question is how low do the seamasters sit in the water? I am trying to design the engine mounts and estimated the water level up 1.5" from the very bottom. I then gave 1 inch clearance from there. Do you think this will be high enough off the water?

flybyjohn

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I don't know why I am so dense. I just did some rough calculations and it seems like 1.5 inches deep in the water would displace 5.7 lbs of water and 1.75" will displace 7.5 lbs of water. If the total weight of the plane is 6-7 lbs then it should sit in the water between 1.5 and 1.75" deep.

So still would like to hear what you all think on the 2 25's. Will they be enough? and will 1" prop clearance above the water be enough?

MajorTomski

I can only comment that the Hobbico twinstar on .25LAs flew great! You're not going to have a pattern plane but it should fly with some authority.

I'd be concern with spray off the bow hitting the engines.

PeterC

I'm not familiar with ASP 25's, but as with any other twin be sure the engines are reliable, especially in the mid range where most of the flying will be done. I used a pair of old HB'25's on a twin seaplane about the same size as the Sig sealane and it had plenty of power so your 25's should be fine. My current project is a Sea Cruiser (designed for a single 40/45) and I am using a pair of OS 26 four strokes. From my experience it should have lots of power.
Peter

JimCasey

My Feedback: (1)

Peter-My HB25 was my favorite engine ever.

Twin 25s on a seamaster should be fine. twin .40s should provide unlimited vertical. But you should raise them a little to keep them out of spray, and unless you have cleverly arranged for one of the motors to have reverse rotation, the right one should be toed out 6 or 7 degrees.

Twin electric with oppositely rotating props totally rocks...And you can slave the throttle off the rudder channel to give you both positive water handling in crosswinds AND the ability to do some RADICAL maneuvers.

PeterC

Hi Jim,
I don't know about the extreme out thrust; I have never needed, or felt a need for anywhere that much. My usual proceedure is to err on the side of out thrust; usually never more than a degree or so. As far as setting the thrust line and the nacells I make the entire nacell sit on the upper surface of the wing with the thrust line slightly above the top camber of the airfoil. With the smaller diameter of the props, compared to what a 40/45 swings, the water clearance is usually fine. Usually my only problem is spray from the hull entering the prop arc. Spray rails usually solve this.
Peter

Wayne22

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You're finally starting on it!!! Good for you... I have a bunch of projects to do before mine sees the light of day....

flybyjohn

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Thanks for all the info. I am using the asp 25's because they are only 37.00 each. Hope I get 2 that run fairly reliable. They are the same thing as a magnum 25 xl not the xls. most of the reviews of the xl seemed ok, the xls was super. The thrust line of the 25's will be about 2.5" to 3" lower than of the engine pod design. I am hoping that it will fly well. I have already put the engine pod suports in the fuse before I decided to go with twins so if I build another wing, I could still put one big engine on it. Will 1/8" birch ply for nacelle sides with 3/16" firewall be strong enough to hold a 25 engine? I will glue each nacelle side to a rib side and the main spars (top and bottom) will go through the light ply.

PeterC

Don't let price be the governing factor although there are plenty of good, inexpensive engines out there. I would suggest a simple single engine test bed of a plane to be sure the engines are reliable. Perhaps the most important consideration is the location of the fuel tanks. the top of the tank should be at, or just slightly above the C/L of the needle valve. If it is too high the engine will flood when it is full and starve when it is nearing empty. If it is too low it will not draw fuel properly when taking off and starve when it is nearing empty. Another hint, use OS A3 glow plugs. They are hotter than normal plugs and are more reliable for use on a twin. don't try to peak the engines out, settle on reliability. Also it is not necessary to have each engine exactly the same RPM wise. 2-300 RPM difference is OK. You can compensate for any variation with the rudder.
Peter

Ed_Moorman

My Feedback: (1)

PeterC,

Back when RC Report was a print magazine and I wrote a monthly column, my flying buddy, Carl "Flaps" Laffert, I did an experiment on out thrust. My test plane was a Goldberg Tiger 2 (.46 size). We added 2 nacelles and covered over the engine compartment. Power was 2 OS .46 AX's. The nacelles were built with 8 degrees of out thrust. Total weight-8.5 lbs.

I realize that 8 degrees looks huge and most people would think that you'd lost a large amount of forward thrust. I did the math and the cosine of 8 deg. is .99027, meaning you have a tad over 99% of your forward thrust. I confirmed this in flight to the best of my ability.

For testing, once the plane was trimmed and I had gotten used to it, I filled the left tank 50% full and the other, all the way. Then I cranked and took off and flew my usual aerobatic flight. The idea was to see how out thrust helps without knowing when the engine will quit. When the left engine quit, I didn't notice anything except a loss of forward speed. No yaw, no nothing. Next, the right tank was filled half way and the left all the way and the test repeated. Same results. Out thrust worked great.

At a later date, I built a Twin Stick ARF, also with 2 OS .46 AXs and 8 deg. out thrust. It turned out a pound lighter at 7.5 lbs. It would easily do any acro I wanted on a single engine.

I will admit, the looks of 8 deg. out thrust will scare you, but after several planes using this amount, I am convinced.

JimCasey

My Feedback: (1)

Like Ed says, angling the thrustline actually equalizes the thrust.  With both motors turning CW when viewed from the rear, the right side of the prop gets a bigger bite.  On the left engine this is almost at the aircraft centerline so the left engine needs little if any out-thrust.  The right engine's thrust is being made mostly on the outside of the nacelle.  This gives it a LOT of leverage to pivot the nose to the left.  On the nose of the plane you'd use 3 degrees right thrust.  Out on the wing, you need a lot more to cancel the left-turning moment.  Ir counter-rotating props, which is why they exist. 

As I remember, The HB and some other motors have a bolted-on front section that can be rotated 90 degrees for reversing the rotation.  Looks funny, though.  The left motor should turn CW (when looking forwards with the plane) and the right motor should turn CCW. 

The brits tried the P-38 without counterrotating props and found it be a completely unsatisfactory airplane. 

Carlos Murphy

My Feedback: (72)

I used Thunder Tiger 36's on my Sea Master twin.

BobieC

John

i love Thunder Tiger 36 pro's i can't rember the exact figure's just know but because the 25's and 36's use the same crank case the 36 is lighter and about .25 hp more for the same size so they fit in were a 25 will go also not a bad engine for the cost

i have never seen 8 degress and when one engines quits it's time to land not continue with a program you do not know why one engine has quit

alasdair

My Lomond O/D twin has a wingspan of 74" and is well over-powered by a pair of Irvine 36 engines.
The model flew previously on a single Jen 56 two stroke and was overpowered, but I used the 36s as I didn't have a pair of 25 engines, which would have been better.
I fly at sea level, so if you fly at 5000 feet it may be different.