Calc for remote steer is different than tiller..

Factor in feet

22•8.5=187

Remote steer uses multiplier of 2 less 90

2•187 = 374

374-90 = 284 hp

The HP rating you cited is for tiller.

Of course, this formula is a simplified method that does not account for the hull. So don't go using it anyone!

BarraMan wrote: ↑Thu May 18, 2023 1:44 am I am NOT a naval architect! and I am confused.

If I use the Australian design standard at the time I built my boat, AS 1799-1.2009, which I understood at that time to be essentially the same as the USCG/ABYC standard, to calculate the recommended max hp for my boat (MG20 x 1.1) I come up with 269 hp - rounded out to 270 hp! Jacques specified max hp for the MG20 as 250 hp, so in the same ball park. I went with 250 hp for my up-scaled MG hull cause I figured that was heaps for my purposes and matched the Yammy 250 Vmax SHO that I had in mind for my build.

While the PH22 is a different design from the MG20, the dimensions for the calculation of recommended max hp as per AS 1799 are essentially the same as for my hull, so AS 1799-1.2009 would suggest 270 hp max. Interestingly, Jaques recommends 150 hp max for the PH22, although he does say that the USCG/ABYC specs would indicate more. I assume that his recommendation is based on the style of the hull and the specified scantlings and glassing etc.

However, if I use what appears to be the latest AS 1799 (USCG/ABYC) standard (AS 1799-1.2021) the calculation comes out as 125 hp for both boats.

Something seems wrong here!

While 125 hp may come close to Jacques recommended max 150 hp for the PH22, my boat with 125 hp would be a 'dog'! My boat handles the 250 hp with ease and would be quite happy with 300 hp. On the other hand, the PH22 with 250-270 hp would be a beast!

The other test for determining max hp specified in AS1799 involves running the hull through a 90o turn at max speed within set dimensions. I am pretty confident that my boat would pass this test - but I am equally confident that a PH22 with 250 hp would fail. I think the PH22 would power slide, or worse, outside the required specs!

So, [0.8 x (22' x 8.5')] - 25 = 124.6 hpThe 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

(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.

Table 183.53—Outboard Boat Horsepower Capacity [Compute: Factor=Boat Length X Transom Width]

If factor is over 52.5 and the boat has remote steering and at least 20″ transom height, horsepower capacity is (raise to nearest multiple of 5) = (0.8 X Factor) −25

Am I going wrong somewhere?