atomic 4 feasibility

[haworth1967]

Hi guys/gals. I have a small 13' dual cockpit aluminum runabout. ('53 Lone Star Admiral) The hull is flat at the stern, so I am assuming that it is a planing hull. Originally it was set up as an outboard, but I have recently come across an opportunity to buy a 30hp Atomic 4 direct drive inboard. The possibility of converting my little boat to an inboard is very appealing, as the boat is driven from the rear cockpit, and with fuel tanks and an outboard hanging off the transom it really sits low at the stern. (not to mention the obvious coolness factor of having it as an inboard launch/runabout!) I have not been able to get a definitive answer as to whether a 30hp Atomic 4 would have enough thrust to plane my boat, or is it just good for pushing around a displacement hull sailboat when no wind is available, so I have come to the experts for some sage advice. My idea is to convert the forward cockpit to an engine compartment, and fabricate an aluminum cover to match the barrelback design of the boat. I believe originally my boat was powered with a 25hp Johnson outboard, but I'm not sure if it was ever a "fast" boat, or just a picnic cruiser. Any advice/guidance would be greatly appreciated!
Mike.

[Jango]

Post a picture of the Boat/bottom.

If the total weight w. motor, fuel and Driver is known, along with H.P., max Speed can be calculated fairly accurately.

Jango

[haworth1967]

Thanks for the reply. I have attached some old sales brochures and a couple of pics (not of my boat, but same hull/model) you can see the specs for the semi-v hull in the brochure. I have been told that the weight of the Atomic 4 in question is 335 lbs. If you can do a quick calculation for me with this info that would be great! (ballpark guesstimate is fine!) Thanks much!
Mike

[FAST FRED]

The Atomic 4 may have been advertised at 30 hp but in most boats they ran at 10.

Why put a 10 hp 300lb motor into a lightweight hull when an outboard would be lighter , faster and leave more room in the boat?

I have a Feathercraft 12.5 Deluxe Runabout , and a 14ft Vagabond and they run fine with 10 to 35 hp motors.

A 4 stroke 20 hp would be quiet and fuel efficent , although far from cheap if new.

FAST FRED

[haworth1967]

Hey Fred! Okay, youve convinced me! I didnt realise the A4 ran at such a low hp. I had heard that you couldnt trust the hp ratings, but wow, when youre right, youre right! A 300 lb. 10hp motor would be crazy. I just found those catalogue shots on the yahoo lonestarboat group. I believe one of them says my hull is rated for a 40 or 45hp motor. If you look closely one of the shots shows an old Merc mark55 hanging on the back. Think I'll look for one of those! Great to hear from a Feathercraft owner! They made some really great little boats! Thanks for the nudge in the right direction.
Mike

[Jango]

Mike, Take Fred,s advice and stay with 35HP or less, perhaps 40 max. A 20HP 4 stroke would also be my choice - far more fuel effecient and quieter.
The bigger motors will not be too friendly to a 50 year old transom.

Jango

[moTthediesel]

I think Fred's point is that while the Atomic 4 will make close to 30 hp, you'd have to be beating the royal wee-wee out of the old lump to do it. In their typical sailboat aux use they were throttled way back and so lasted just about forever. Try shooting around all day at 3500 however, well, all bets are off.
If you want to spend a lot of money for modern power then, as said, a 4 stroke OB is deff the way to go. For very little money though you could find an OMC "Big Twin" tin top like that Johnson in the pic you posted. Any flavor you prefer (Evinrude, Johnson, or [my fave -- my Dad sold them] Gale), those motors are cheap ($400 for a real good one), plentiful, and harder to kill than a box of rocks. Are they thirsty? You bet! But you'll fill a lot of "Cruise-A-Day" tanks (as in; HUNDREDS) before you flatten your wallet to the tune of a new 4 stroke.
An old 4 banger Merc would be faster, and have more bling value, but in my experiance they're way harder to keep happy.
moT

[haworth1967]

You guys crack me up!!! Point taken though, the A4 is a bad idea. This is a great site with REALLY knowledgeable people. I have an old Evinrude 35hp Lark for it, but the bling factor is important as I plan to take it around to some boat meets next year. Primarily its going to be a working boat however, cause theyre just not much fun on a trailer! I think some sand bags in the bow and a big ole vintage Merc is the new plan. The transom has been rebuilt with some very expensive marine plywood, but I have a little skill with metal fabrication, so I may go ahead and build a small aluminum cage in the rear compartment that hugs the sides of the hull, and connects to the bulkhead behind the drivers cockpit to better distribute the force a 45+ hp motor will generate. If you can believe the brochures, the thing appears to have been a pretty fast boat for its size/weight. I'm sure having a lot of fun making it seaworthy again! I will try and post some pics on here as the project progresses.
Mike

[moTthediesel]

Mike,
Don't get me wrong, I love the old Mercs too, I've owned a white 700 six on a Glaspar G3 for about 35 yrs. Still runs like a watch. If you get a good 4 banger, keep it tuned and give it clean gas it'll work for you. The older ones with the dry exhaust scream like the banshees of hell, but that's all part of the fun.
Be carefull of the sandbags though, they're fine in calm water, but they'll flex the hull in an unnatural way in a chop. That was one of my Dad's pet peeves with people he sold his aluminum (Cherokee) boats to. I remember asking him how putting sandbags in the bow is any different than having someone sit in the front seat? His answer was that the sandbags will never yell at the helmsman for knocking the fillings out of their teeth. The front seat passenger then acts not only as trim balast, but also as a kind of "wave impact govenor" in a small aluminum boat.
For trim problems, IMHO simple solid trim tabs would be a better solution.
moT

[marshmat]

A 300+ lb. 10hp motor would be crazy

So would a 300+ lb 30 hp motor in a boat like that... my Johnson J30 weighs a bit over half that.
Looking at your boat I agree that 30 hp is around ideal. The boat was designed for an OB and will work best with that. But remember that it was designed for a '50s OB, and motor weights have gone up since then, so keep an eye on the weight and be prepared to brace the transom if necessary.

[gonzo]

You can get 27HP out of an Atomic 4 for intermitently. It actually weight 437lbs. It is a very old and poor design. For example the crankshaft has only two bearings, one on each end. It flexes badly and wears out bearings. It is widely used in sailboats where it was usualy rated at 20HP and normally run at half speed.

[LP]

What condition is the A4 in?