Re: [bolger] Topaz
Brad, my apologies! I have always confused Topaz with Champlain, but I
admire your work even more now. 25 mph on 75 hp in a 31 footer is
remarkable, regardless of the angle.
My remarks are probably only relevant to the overpowered runabouts flooding
the market. You probably don't have enough excess power to see much result
from the power trim at speed.
Don Hodges, who is going to either shut up or put "I'm Sorry!" in my sig...
admire your work even more now. 25 mph on 75 hp in a 31 footer is
remarkable, regardless of the angle.
My remarks are probably only relevant to the overpowered runabouts flooding
the market. You probably don't have enough excess power to see much result
from the power trim at speed.
Don Hodges, who is going to either shut up or put "I'm Sorry!" in my sig...
> Don, I must defend my project. The topaz is 31'. I feel better now.
If memory serves correctly the optimum planing angle found in Lindsay Lord's
"Naval architecture of Planing Hulls" is 2 to 2.5 degrees bow up.
hal
"Naval architecture of Planing Hulls" is 2 to 2.5 degrees bow up.
hal
>
>> I have a question of those more familiar with speed boats. A couple of
>> nights ago, Bolgers called and asked if I could get together a video of
>the
>> boat. This morning, my wife and I went to the lake and did just that.
>> However, upon veiwing the boat under power, I was surprised. In the video
>> at plane the bow was completely out of the water with the boat planing on
> > only the flat 1/2 to 2/3 of the hull
>
Brad,
Sounds about normal for a modestly-powered 22 footer. In the real
high-speed planing boats (like my 150 hp 18-footer), the technique is to
trim the boat level (motor trimmed down at stern) until the boat is on
plane, then power-trim the bow up (motor up at the stern) to minimize the
planing area and increase boatspeed/engine rpm. If the bow-up trim is
dialed in before applying power, the boat tends to "mush" onto plane instead
of rising smoothly. If you don't have power trim, pick the highest motor
trim which gives a smooth rise to plane, and just live with the
less-than-optimum speed/rpm on plane. Your narrow, relatively light 22
probably is better-behaved as to "mushing" up on plane, so you can trim up a
bit more than a heavy, wide conventional runabout with fixed trim.
Don Hodges
Sounds about normal for a modestly-powered 22 footer. In the real
high-speed planing boats (like my 150 hp 18-footer), the technique is to
trim the boat level (motor trimmed down at stern) until the boat is on
plane, then power-trim the bow up (motor up at the stern) to minimize the
planing area and increase boatspeed/engine rpm. If the bow-up trim is
dialed in before applying power, the boat tends to "mush" onto plane instead
of rising smoothly. If you don't have power trim, pick the highest motor
trim which gives a smooth rise to plane, and just live with the
less-than-optimum speed/rpm on plane. Your narrow, relatively light 22
probably is better-behaved as to "mushing" up on plane, so you can trim up a
bit more than a heavy, wide conventional runabout with fixed trim.
Don Hodges
> I have a question of those more familiar with speed boats. A couple ofthe
> nights ago, Bolgers called and asked if I could get together a video of
> boat. This morning, my wife and I went to the lake and did just that.
> However, upon veiwing the boat under power, I was surprised. In the video
> at plane the bow was completely out of the water with the boat planing on
> only the flat 1/2 to 2/3 of the hull
In a message dated 7/26/00 5:26:31 PM Central Daylight Time,
sailormon@...writes:
<< However, upon veiwing the boat under power, I was surprised. In the video
at plane the bow was completely out of the water with the boat planing on
only the flat 1/2 to 2/3 of the hull. ... ... Is this a normal thing in
flat bottomed speed boats? >>
Absolutely normal. When planing the bottom of the boat has to have some
angle of attack - i.e. rising from transom forward - to generate the dynamic
lift that is "planing". Most, perhaps all, powerboats designed for serious
planing performance have dead flat buttocks aft. - thus a 5 degree angle of
attack causes the boat to rotate 5 degrees from horizontal. A boat with
buttocks rising aft must rotate to a correspondingly greater degree to
achieve the same angle of attack. If the buttock angle is too steep, it will
just want to stand up on its transom, given enough power (i.e excessive for
the design). Weston Farmer designed a boat called "Trumpet" which is
featured in an appendix to "From My Old Boat Shop" which used a feature he
called "decalage" to produce a boat that would actually run level, or nearly
so when planing. To describe the concept crudely, it is as though the boat is
"hogged" in the middle, so that the aft 1/2 of the hull incorporates the
angle of attack when at rest. He claimed that the boat performed very well.
I've noted Bolger's claim for small trim changes with speed increases on
"Sneakeasy" and its progeny, but I don't think that the claim here is that
they run "level" at all speeds. (I'm speculating here, however, as my
experience is limited to thoroughly ordinary production speedboats built
between the 1940's and the near-present.) I think this is only by comparison
with short, wide, heavy (i.e. typical production) speed boats. These almost
universally have to assume a very high angle of attack to get "up" on the
plane, or to maintain speeds below their minimum "efficient" planing speed.
(and dragging a great wake in the process) Given enough power, they get "over
the hump" when they reach a high enough speed to generate adequate lift with
a lower angle of attack and, if not poorly designed, will ride relatively
"level" (i.e. small angle of attack) once they get up to speed. Below that
speed their performance varies from less-than-optimal (below hull speed) to
abyssmal (above hull speed and below efficient planing speed). In the case
of Bolger's boats I think the claim is that the boats perform smoothly
throughout their speed range, without having to assume any high angles of
attack at any intermediate speed. For what its worth, the less hull in the
water at planing speeds, the less skin friction to be overcome. Some
super-zoomy bass boats are (or once were) designed with a short, full width,
flat spot just ahead of the transom (a "planing pad"), with the intention of
reducing wetted area (at top speed) to a minimum. They were supposed to plane
on just the first couple of feet of the hull ahead of the transom, with the
rest of the hull in the air. This was only possible on calm water with
obscenely powerful motors (175 -250 hp on 16'-18' open boats) , and I think
power trim had to be manipulated as well.
Ciao for Niao,
Bill in MN.
sailormon@...writes:
<< However, upon veiwing the boat under power, I was surprised. In the video
at plane the bow was completely out of the water with the boat planing on
only the flat 1/2 to 2/3 of the hull. ... ... Is this a normal thing in
flat bottomed speed boats? >>
Absolutely normal. When planing the bottom of the boat has to have some
angle of attack - i.e. rising from transom forward - to generate the dynamic
lift that is "planing". Most, perhaps all, powerboats designed for serious
planing performance have dead flat buttocks aft. - thus a 5 degree angle of
attack causes the boat to rotate 5 degrees from horizontal. A boat with
buttocks rising aft must rotate to a correspondingly greater degree to
achieve the same angle of attack. If the buttock angle is too steep, it will
just want to stand up on its transom, given enough power (i.e excessive for
the design). Weston Farmer designed a boat called "Trumpet" which is
featured in an appendix to "From My Old Boat Shop" which used a feature he
called "decalage" to produce a boat that would actually run level, or nearly
so when planing. To describe the concept crudely, it is as though the boat is
"hogged" in the middle, so that the aft 1/2 of the hull incorporates the
angle of attack when at rest. He claimed that the boat performed very well.
I've noted Bolger's claim for small trim changes with speed increases on
"Sneakeasy" and its progeny, but I don't think that the claim here is that
they run "level" at all speeds. (I'm speculating here, however, as my
experience is limited to thoroughly ordinary production speedboats built
between the 1940's and the near-present.) I think this is only by comparison
with short, wide, heavy (i.e. typical production) speed boats. These almost
universally have to assume a very high angle of attack to get "up" on the
plane, or to maintain speeds below their minimum "efficient" planing speed.
(and dragging a great wake in the process) Given enough power, they get "over
the hump" when they reach a high enough speed to generate adequate lift with
a lower angle of attack and, if not poorly designed, will ride relatively
"level" (i.e. small angle of attack) once they get up to speed. Below that
speed their performance varies from less-than-optimal (below hull speed) to
abyssmal (above hull speed and below efficient planing speed). In the case
of Bolger's boats I think the claim is that the boats perform smoothly
throughout their speed range, without having to assume any high angles of
attack at any intermediate speed. For what its worth, the less hull in the
water at planing speeds, the less skin friction to be overcome. Some
super-zoomy bass boats are (or once were) designed with a short, full width,
flat spot just ahead of the transom (a "planing pad"), with the intention of
reducing wetted area (at top speed) to a minimum. They were supposed to plane
on just the first couple of feet of the hull ahead of the transom, with the
rest of the hull in the air. This was only possible on calm water with
obscenely powerful motors (175 -250 hp on 16'-18' open boats) , and I think
power trim had to be manipulated as well.
Ciao for Niao,
Bill in MN.