top end speed

[kevin]

THIS WEEKEND I HAD THE BRONZE PROP ON MY 16FT. 68 302 DONZI REPITCHED AND CUPPED. IT IS NOW ALMOST A 23. THIS DROPPED MY RPMS TO ABOUT 4400 AND 53.5MPH ON GPS. THIS WAS AN IMPROVEMENT OVER THE 21 PITCH. EVERYONE THAT LOOKS AT A 16FT. DONZI WITH A V8 ASSUMES THEY ARE 65-70 MPH BOATS. MY FATHER JUST BOUGHT A 201 LARSON WITH A 350 AND VOLVO DUOPROP. WITH GPS THIS QUITE, AND PRETTY RUNABOUT TURNED OUT 51MPH AT 4000RPM. WHY IS IT, WITH THE ADVANCED HULL DESIGN, HORSE POWER AND LIGHT WEIGHT, 55 SEEMS TO BE MAX SPEED. I CANT BELIEVE THAT A 90 LARSON COULD EVEN COME CLOSE TO MY DONZI BUT IT DOES. (MINE STILL LOOKS BETTER AND SOUNDS BETTER) AND I KNOW I HAVE AT LEAST AS MUCH HORSEPOWER. ANY IDEAS.

[EricG]

My Smallblock '88 Z21 is getting 59 on the speedo (which I figure is low 50's in reality), which I was also disappointed with. I've realized that in a calm lake with only ski boat wakes to contend with, I'm definitely not the fastest boat out there, however, as soon as the wind blows up some decent chop, I'm still able to run Wide open and get the same speed - I haven't found a Bayliner, Larsen, etc. that can do that yet....I don't know if that is the case in a 16, but I'm sure you'll have a heck of a good time finding out..

EG

[Tony]

Donzis are heavy, and so are V-8's. Also, the deep-vee design is not a pure speed hull...it offers other advantages. There are boats that are faster, of course, but they will not ride rough water as comfortably and they will definately not last 30 years! My '67 16' with a 302 For and a 23 pitch standard aluminum prop runs 58 mph @ 4800 rpm's. Maybe a SS prop will get me above 60 but I doubt I will get too much more than that. 70 mph in a 16' sounds wild...is there anyone on this board that has one?

[HOWARD O]

Kevin, my brother's '69 with the original 302 has about the same top speed. I wouldn't be too disappointed, it's plenty fast enough! Like the others have said, if you want to embarrass your father, take your little 16 footer and race him in some decent size chop! Therein lies the difference!

[Barry Phillips]

Kevin : your top end seems about right,
you did not mention your drive package.
Is it an early Volvo set up or a V drive.
Early Volvo drives lacked power trim
which can increase your top end
dramaticly. I have a 98 18' Classic and
with only 250 hp will run 65 mph,
confirmed by radar. If I run trim in, I will
lose 5 mph, so airing out the boat makes
a lot of difference, your father's boat is
is more than likely has a stepped bottom
and I know it has power trim, this
makes a big difference. A new 302 Sweet
16 can run well over 60mph dee vee or
not.
SO-SLO

[GeneD]

These kind of things are very fickle aren't they?
My bud just finshed building a 454 for his center console 28 something or other. He spared no expense. He even has these tricked out 427 heads on there. Some kind of special marine cam with .500 something lift, roller rockers, ect.
He bought a new gimble, transom plate, Bravo One drive, propeller, the works.
The thing just barely goes over what it used to do. He has gained a little over 450 RPM's, it's up to 4750 now. And a top speed that is unknown at this point. But the general consensus was that it definitely was not as fast as was expected, and barely faster than the stock 454.
He is disappointed, his wife is disappointed, the rigger is disappointed, and as an outsider that watched the whole thing, I'm disappointed.
Just goes to show ya, you can never tell what is going to happen, no matter how well planned these things are, you can always get let down.
I realize that this has little to do with the top speed of a 16 Donzi.
I too was let down when I put the 350 (from a 305) and the Alpha SS (from a stock Alpha One) and lost 4 MPH, to a top speed of 51 MPH. It wasn't until I re-geared the little wonder and got a new prop that I realized the top speed potential of my hull and motor/drive combo.
It just doesn't always have to do with HP, many factors make up a fast boat.

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GeneD
007
Melbourne, Florida

[PaulO]

As Gene has stated, there are many factors that determine top speed. I too am amazed recently at the generic runabouts that are doing mid to high 50s with small block chevys supposedly confirmed by radar and reported on in many recent issues of boating magazines. I should find the time to look back through some past issues because I recall several that were in the 21 foot range and had some decent deadrise also, indicaing that rough water capabilities might not be too bad. The one thing I can't figure out (not being a marine architect) is why it seems that many longer boats are capable of faster top speeds. There are longer boats than my 18 with similar power that are faster. with all the effort to "air out the hull" and diminish the wetted surface, it would seem a longer boat would suffer. Not to mention additional weight. What are the thoughts on this? Or, is this just another case of "top speed exaggeration"?
PaulO

[Craig]

Kevin,
As I've looked at this top end speed thing (and, read, and listened, and tried different props, etc., etc.), one big thing that effects this is the outdrive's gear ratio. My old Volvo 270 has (I believe) an approx. 2:1 (Old 270's had 3 -- see Volvo home page). Even with a very high pitch prop it only gets to mid 50's (Boat is 16' w/ 351 -- 290 hp). Depending on what gear ratio you have it will greatly effect top speed. I have a friend who has a big ole' 20' Chaparral (probably between 3,000 and 3,500 lbs) that runs nearly 55 mph w/ a Chevy 350 (not E.F.I.---supposed to be like 260 hp) and 21" or 23" Stilletto prop. It is surprising, but again, the Merc. Alpha drive's gear ratio ( I think 1.5:1) helps a lot.

[Blewbyu]

Kevin and PaulO- I think there is something to the OAL (overall length) thing.At least with displacement hulls-more waterline length equals more speed....I believe.Don't know if the equation tails off,would think that total wetted area friction would eventually take any benefit of OAL.Witness sailboats (I'm sorry) of a length limited by their class- the designer bulges them out in the middle section to increase their waterline length. WWII destroyers were long and thin for same reason- faster w/same horsepower. I don't know if all this applies to planing boats, especially Donzis, which are probably airborne more than in the water.....bottom line is, an 18 is faster than a 16, and a 22 is faster than both, all with same hp.
Regards- Uncle Jeffrey

[RickR,GroveCity]

bluebyu
I beg to differ, if your analogy were true we'd be installing 4.3s in 80' Magnums (instead of twin 1700HP Cats) and they would run 70 MPH.
Speed with a planing hull is determined by hp, wetted surface, hull configuration, wind resistance, weight, etc...
The runabouts that have good top speed numbers probably have less deadrise, wider(and or reverse) strakes and or a pad.
Classic Donzis have a 24 Degree deadrise at the transom and a VERY fine entry, this hull is a very slow design but sure gives a nice ride in the chop. If you want them to perform you have to add lots of hp.
BTW;The 22 Classic has a reverse chine which helps a little on top end.



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RickR mailto:riggerb@aol.comriggerb@aol.com</A>

[PaulO]

Rick,
What exactly is a reverse chine? I know what a chine is but, if I am correct about what a reverse chine is, I can't see how it helps top speed. Thanks
PaulO

[Blewbyu]

RickR.........GOTCHA! Ah SEEN it wit my own feeble eyes........
Last August,Seafair Races at Seattle.Don,t see Donzi's all year on Lake Washington- then hoards of 'em show up for Unlimited Hydro race.Immediately after the races-thousands of boats take off for home and churn the lake into a maelstrom! This is when the Donzi's get the REALLY BIG AIR!!!Spectacular stuff! After that showtime- four of us went down the East Channel, and three lined up for an impromptu 1 mile race- a 1976 16',1973 18' 2+3,and a 1979 22' classic, all 3 had Merc 350/300 motors w/less than 100 hrs and no mods.All had Alpha 1 drives,18 and 22 w/same size props, and 16 had 1/2 more pitch and 1/2 more dia than the other 2.The 16 came outta the hole much faster, but the others ate him in the long run.22 won by 2 boat lengths over the 18, and the 18 had the 16 by at least 8 boatlengths. All three trimmed all the way in for holeshot, then each made their own trim and tab decisions.All 3 boats had clean, unwaxed bottoms and medium cups in their props according to their owners.While I think I qualified my earlier statement by stating I thought the waterline thing applied to "displacement" hulls- I believe as you do that prop rpm is directly related to hp/torque/gearing/etc,etc-and SPEED (Amen)is the final result.How would YOU choose to explain that impromptu race with the 22/18/16 finish in that order? The spread between the 16 and the 18 rather surprised me more than the order of finish.Tuning differences? Whaddya think? I'm confused...


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[Barry Phillips]

I ran with a bigblock 22 last weekend at
Lake George in my 18 he was single, I
had a passenger. Top speeds where
similar. The point is in medium chop
and wakes, I could not make my 65mph
top end , just to hairy, got to much air.
The longer lenght of the 22 just let it span
the waves better. Even if I had more HP
the boat would fly to much, uncontrollable
Lenght dose count for contollability.
SO-SLO

[PaulO]

Barry is accurate there! I had the same experience at lake george. Follwing Sunday's festivities, I headed south on the lake side by side with Fast Eddie. He in his 22 and I in my 18. Although I can't say where Ed was as far as throttle settings, I can tell you that I could not run flat out because of the chop and wakes while Ed could maintain a speed faster than mine ( although not neccessarily faster than my top speed).
It looked from my vantage point that Ed was having a nice smooth ride where, I had to run on the edge of control to keep up. I guess I need a 22.
PaulO

[GEOO]

PaulO, A 22 is more stable and can take rougher water then a 18.
But don't forget Eddies 22 has a 502; trimmed out it will run in the low 70's. He can run the hull wet & stable, (trimed in) and still hit the mid 60's. While your boat was probably trimed out for more speed. GEOO