TOPIC: Motor size for 1961 Larson

I recently purchased a 1961 Larson 16'. I have been trying to determine the recommend motor size for this boat. During the rebuild I and trying to keep most things traditional but with the engine I want to update with a 4 stroke outboard. I am thinking somewhere in the range of a 40 to 50hp.

Can anyone point me in the direction of hull specs and recommendations for this hull or has anyone repowered a similiar hull and have recommendations on size and hp.

Thanks
Randy

Is that the Larsom All-American? I have a 1960 All-American powered by a Evinrude Fat Fifty. I beleive they will handle up to a 75hp. Is it a 15" or 20" transom?
Power it with a 61 or 62 4 stroke Homelite 55hp.
I have some specs around here somewhere just can't find them.

I not sure about whether it is an All american or not. The guy I purchased the boat from did not know much about it. I actually discovered it was a 61 when I went to register it and they had it listed as a 61 instead of 68.

The hull is great shape and all the deck hardware is good but it requires new interior, motor and little details.

I have been reviewing the web trying to determine the different models and options for this boat but have not found much in the way basic specs and a major lack of interior pics.

Is there a way of determine the model type?

Bunches of Larson brochures right here on this site in the Classic Library section. The Larson Boat Company also has a bunch of their old brochures posted on line.

There was a vintage Larson Boat website at one time, but not sure if it's still around.

Don't pay much attention to seller's memory or paperwork such as registration, title, bill of sale... regarding old boats. They are notorious for being inaccurate.

Andreas

If it is about 14' long it could be a Playboy. 16' is an All-American. Playboy 18-80hp and a All-American 18-90hp.

It is very easy to calculate the max safe horsepower.

The USCG formula is centerline length times the maximum transom beam. Then if the boat has twenty inch transom height (19 or more is considered 20) and remote steering ( wheel or stick steering) the it's the length times the transom width time 2 minus 90. The round to the nearest multiple of 5. ( i.e. if it were 62 you would round it to 65)

See newboatbuilders.com/pages/hp.html

Not saying your wrong with that formula or the USCG..... I would have to think there are some exceptions to the rule. I see 18' bass boats with 200 hp hanging on them.Then I see where some 18s have a 100hp max,something is not adding up. Even similiar sized older boats here have a huge difference in what they are rated for.

We put a 65HP Merc on our 16' Larson Thawk, it's rated for a 60hp. Boat runs great, wouldn't want anything less and doesn't need any more. My 2 cents

Mr. 88, well if the formula is wrong then I have been doing it wrong for over 20 years. For 20 years I worked in the USCG Office of Boating Safety enforcing boat manufacturing standards. Yes you will see some boats with really big engines because the owners don't particularly care what the label says. For any 18 foot monohull boat to be rated for 200 HP it would have to have an 8 foot wide transom. Once you hit 20 feet though the formula no longer applies. But the law applies to boat manufacturers. It is not a violation of Federal law for an owner to put a bigger engine on his boat. But it may violate the law in some states and will surely get your insurance canceled.

Here is the Federal regulation: Unfortunately the chart didn't come through so well, but you can see it at the link I posted earlier.

Subpart D--Safe Powering
Sec. 183.51 Applicability. This subpart applies to monohull boats less than 20 feet in length, except sailboats, canoes, kayaks, and inflatable boats, that are designed or intended to use one or more outboard motors for propulsion.

Sec. 183.53 Horsepower capacity. The maximum horsepower capacity marked on a boat must not exceed the horsepower capacity determined by the computation method discussed in paragraph (a) of this section, or for certain qualifying boats, the performance test method discussed in paragraph (b) of this section.
(a) The maximum horsepower capacity must be computed as follows:
(1) Compute a factor by multiplying the boat length in feet by the maximum transom width in feet excluding handles and other similar fittings, attachments, and extensions. If the boat does not have a full transom, the transom width is the broadest beam in the aftermost quarter length of the boat.
(2) Locate horsepower capacity corresponding to the factor in Table 183.53.
(3) For a boat with a factor over 52.5, if the horsepower capacity calculated in Table 183.53 is not an exact multiple of 5, it may be raised to the next exact multiple of 5.
(4) For flat bottom hard chine boats with a factor of 52 or less, the horsepower capacity must be reduced by one horsepower capacity increment in Table 183.53.
HORSEPOWER CALCULATIONS FOR OUTBOARD MOTORBOATS
Outboard Horsepower Capacity
 Compute the Factor = Boat Length x transom width
 If Factor is (to the nearest integer) 0-35
36-39
40-42
43-45
46-52
Horsepower is
3
5
7.5
10
15
Note: For flat bottom hard chine boats, with a factor of 52 or less reduce HP one capacity increment
If the Factor is over 52.5 and the boat has :
Remote Steering and at least 20" Transom Height
HP =
(2 x Factor) -90

No remote steering or less than 20" transom Height
 
For flat bottom hard chine boats
HP =
(0.5 x Factor) -15

For other boats
HP =
(0.8 x Factor) -25

In my first reply my first sentance was I am "not saying your wrong..." I was wondering if there were any exceptions to the rule, So please read again before jumping down my throat.You now state the exception is the lenght of the hull.That was not mentioned in your first post.Thats all I was asking about. Not questioning your 20 years of experience in the USCG....or anything else to do with the formula of rating HP.

Randy, I just went through the same exercise with my Catalina. You might want to check the thread "Power needed for a Dorsett Catalina" from a couple of weeks ago.
The problem is, you have to worry about weight as well as HP. Back in the day, you could get 75HP and up in a motor that weighed less than 250 lbs. You can still do that today if you want to run a period motor, but you mentioned that you wanted a 4-stroke and the power/weight ratio on those is a lot less forgiving.
IMHO, I think the driving factor should be the weight. After all, the naval architect that designed your boat laid out the lines based on a set of assumptions regarding weight loads and their placements on the boat. To me, those design factors should be respected.
My boat says it can handle 150 HP and period motors were around 250 pounds so that became my number one constraint:
* < 250 pounds
* > 50 HP
* As new/reliable as possible
* As cheap as possible
What I found to be the best compromise between these four was one of the post-2004 two-strokes with direct oil injection. I ended up with a Yamaha because I liked the maintenance history on that particular motor, but Mercury made these types of motors, as did others.
Anyway, that is what I did. Best of luck on your search!
Bill