Underpowered mac 19?

[magnetic]

I am not too hot on displacement hull speed calculations and the need to cube power in order to double speed through the water, but I do know that my boat won't get up on the plane and I'm not sure why.... Alex seems to have some insight into this, though, to be frank, whether or not the Titanic could have achieved 6 kts against wind and tide using a hand-held fan doesn't really tell me much about getting a up on the plane with a wrongly-propped 25hp Yamaha.

The standard prop for my engine is 9 7/8 " x 10 1/2 " and I have just ordered a replacement 9 7/8" x 8"; there is some concern that this may cavitate badly, in which case other options for this engine are 9 7/8" x 9 and a Dual Thrust 10 1/4" x 8 1/4"

At the end of the day it will probably be a case of getting out on the water with a set of props and a tacho and seeing what happens

I'm grateful for your help, but as I am neither going to rush out and buy a 90hp engine or a 26 or , I guess I'll just have to experiment a bit

[Frank C]

... I guess I'll just have to experiment a bit

Bravo!
And best of luck.

. . . It's good that you have that tach. Max rpms, plus a GPS to gauge your speed, are going to be your best metrics on performance of your specific hull/motor package. Sounds as if you're well-experienced to judge whether your motor is overpropped, in which case you'll need to try a different prop.

Most here have tested from 3 to 6 props, looking to optimize their particular hulls with various outboards. Since the Mac26 can just barely plane at 15 knots with a 50hp, it seems totally unlikely (to me) that your 25 has any chance of reaching 13 knots.

[Highlander]

Magnetic

Did you not get my pm . I just raised my eng 1" so now the cavation plate is level with the bottom of the transom it used to be 1" below this will now increase my top end speed but in doing so I lost more trim in angle so now I'll have to install the wedge kit witch I needed anyway as I could not run anymore than 5mph in rough seas due to not being able to get that bow down in slower speeds. With the semi V full hull & hard chines this boat will run hard in rough sea's with more inboard trim & plane also, I've buried this boat in 15ft swells under sail heeled at 25-35 deg. shes a tough little sucker when heeled over that far she's basicaly running on the hard chines a deep V And just digs in Its a little bit exciting to say the least
they only made about 1000 of them I believe so they are a hard bird to find took me 4yrs to find mine
get the mast rake down to 2 deg max to much weather helm otherwise .
I'd say you'll want min 40hp if I was repowering I'd be looking at a 50-60hp I know alot of guy's have the 50hp eng. same eng. as the 40hp just diff eng. ecu allowing higher R.P.M.'s & richer fuel ratio any way she is a frustrating boat to sail untill you learn how to tame her
by the way she has twice the sail area to weight ratio than 26X, 26M eat your heart out guy's & she is way under powered sail wise re my cutter rig conversion Yeh they are very envious of our

Cheers John

[Highlander]

Ray

Your so kind "where's me bottle" laddie

I'll drink to that John

[NiceAft]

Highlander,

Och aye

Ray

[Highlander]

Och Ray

Your becoming a bad influence 'Hick" what was that you said Laddie
three sails to the wind you are envious of three head sails
I heard from a secret source you've got that Niceaft in hiding for fear she's gona get her butt kicked big time from a wee shetland pony mac 19 dream on laddie,

[beene]

I sure hope I don't smoke past you John.

After all that talk.

Maybe I should drag an anchor.




G

[magnetic]

OK - just tried the old prop with a tacho, fitted the new prop and am a verry happy Bunny -

1. Old prop is 10.5 pitch - WOT 3950 rpm, upstream 8.4mph, downstream 9.5 mph (13.4 / 15.2 km/h) (River Thames is in full flood right now)

2. New prop is 8 pitch - WOT 5050 rpm, upstream 11 mph, downstream 12.5 mph (17.6 km/h and 20 km/h), pops up on the plane very briskly

OK, so the Mac 19 is not going to break any speed records when paired to a 25hp Yamaha BUT

at least I now appear to be getting into the correct rev band (4,500 to 5,500), and the boat is level out on the plane at those speeds. To be honest, all I was looking for was 13 mph, so 12.5 isn't bad!

My next steps will be to get the engine serviced in order to see if I can squeeze out a few more rpm, then I'm going to fit a Permatrim hydrofoil to see what impact that has and, if WOT gets closer to 5,500 and/or 25 km/h (due to lower slippage thanks to the foil) THEN, and only then, I might try my luck by pitching the prop up to either a 9 inch or possibly an 8.25 inch Dual Thrust (which has a wider diameter and greater blade area)

In all honesty, if all I have to play with is 600 rpm (i.e. if the newly-tuned engine still only achieves WOT on the 8 inch prop of 5,100 rpm, then I could maybe swing a prop which runs 600 rpm slower and still be within the manufacturer's spec for WOT) this will probably only allow me an extra inch or so of pitch (remember, the old prop was 2.5 inches coarser, and that caned performance lower down), which might only give me 12.5% more top end speed, which is only 1.5mph, so it's hardly worth the investment.

Anyhow, I am grateful for all your comments and advice; the news is - a WILL plane with a 25hp outboard, it's just a little slow

[Highlander]

Na Beene

I'll be watching to make shure that your prop is out of the water other wise I'll be cutting accross your bow dragging a very long line never towed a mac 26M backwards before from the prop that is Now that would be mighty embarising for you the mustang 26M being hauled off by the wee shetland pony mac19

Ray

You've been very quiet of lately

Cheers John

[Frank C]

... IF, on the other hand, 25hp is just fine for, say 13kts, then I would appear to have a fuel/carb/tuning/prop issue with the existing engine - which is probaly a tenth of the cost to fix....

.. To be honest, all I was looking for was 13 mph, so 12.5 isn't bad! ...

A frequent source of missed communications ... assessing speeds in knots vs. MPH.

[magnetic]

What does the word "pedantic" actually mean, by the way?

Earlier in this thread I set out my dilemma -

"my question is really much, much simpler -

IF the current 25hp engine was running just fine AND the prop was right, would it still be incapable of pushing the boat up on the plane?"


to which you kindly replied that

it seems totally unlikely (to me) that your 25 has any chance of reaching 13 knots

OK, here we go, just for the sake of it, let's take it around the dance floor one more time ....

1. I have now established that a powered by a 25hp outboard can get on the plane and has a theoretical maximum speed of at least 13 KNOTS.

I say this is theoretical, because I have not yet actually achieved 13 knots. However, a variety of online calculators tell me it should be possible*, and my own empirical experiences with a de-pitched prop and an over-low outboard strongly suggest that it should be so, if I got everything finely tuned and properly configured. (NB best estimate of theoretical nirvana is around 18.25 MPH).

2. I have achieved almost 13 MPH with minimal changes; this is a good proportion of 13 KNOTS, and I am therefore tempted to sit tight without investing any more time or money. If the extra few mph or so would cost me £300 / $600 then - although theoretically achievable - it would hardly be worth it.

On the other hand, some of the problems with my boat are hardly rocket science, and at the end of the day it's a judgement call which to fix and which to live with.

3. Therefore - and here is the tricky intellectual bit for all you pedants out there- the answer to my original question ("Is it even worth tinkering with the 25hp outboard, or will I end up having to go for a bigger engine anyhow?" is a resounding "YES!"

That YES is the same, by the way, whether you express it in Knots, MPH, KMh, joules, pascales, therms, kilowatt hours or curies...

* http://continuouswave.com/whaler/reference/prop2.html is a useful reference, as is http://continuouswave.com/cgi-bin/propcalc.pl

[Frank C]

Far from pedantic, I had hoped to convey that a "book answer" to your original question simply does not exist ... at least not in these archives. Ergo, your apparent frustration was simply excess tension, to no avail. BTW, I've never detected any disdain whatsoever for the Mac 19. They're simply so rare that most here never had opportunity to learn more.

Even with dozens of Suzuki or Honda 50s on the Mac 26, it seems each boat performs as an individual example. It's not surprising that you might quickly become THE expert on a 25HP for (your) Mac 19. My initial advice, "Most here have tested 3 to 6 props ...," was simply intended to encourage you in that regard.

I'm surprised you've gotten onto plane with the 25 HP, but that's simply my error in judgment, based upon common results for the Mac 26. It's not from a reading (or pedanting) of the non-existent history. Congrats. Though the forecast was wrong, maybe that initial advice wasn't so bad.

The 15% difference between MPH and KTS has frequently been a point of confusion in the past. However you choose to assess that difference ... your own choice. I'll be surprised (again) if 12.5 MPH infers, in fact, a potential WOT of 15 MPH (13 knots), but best of luck.

FWIW, I'd guess that planing fins have an equal chance of slowing you down, rather than getting closer to a top speed of 13 knots. This is probably a pedantic assessment, search on hydrofoil. But your empirical testing, followed by detailed reporting, are welcomed and appreciated ... valuable archives for the next member who searches, or asks.

[Catigale]

I guess Im missing the revelation here...it looks to me like all that has happened is you have replaced an obvious over-pitch prop with one much closer to correct, perhaps even optimal, and are hence seeing performance commeasurate with developing full HP of your 25.

13 mph or 13 kts does not necessarily mean on plane either of course, you could easily still be in transition. I dont know anything about the 19 hull characteristic to be fair.

[Alex]

Magnetic
I don't understand why you bought sail boat !!
If you need speed, buy power boat. To much questions about speed and no any about the sail........ With my Honda 30 hp I have 12 km.h with 2500 rpm and I am very happy. Any way I can do 25,km/h with prop 10x9, but I don't need it Just leave the dock, rise sails and a lot of Fun...

[johnnyonspot]

Magnetic,

I for one appreciate your sarcasm, but being a sarcastic person myself, I know it is sometimes misinterpreted..... or sometimes not?