�� Hull Design �� Buyer guide jetcraft.ru Boat Materials

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Hull Design �� 

Your hull is probably the single most important factor in your boat selection and

enjoyment. All builders have access to the same power, drives and accessories. Pick the

wrong hull for what you want to do and your enjoyment may be (substantially) reduced.

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Hull design also affects your operating cost and your pocketbook. A poor or

inappropriate hull will cost much more to operate than a well designed hull.

Delta Keel

As discussed earlier, delta keels are like hooks in a boat. They generate lift quickly and

get the boat out of the water quickly which can lead to higher speeds.

However, delta, flat bottoms have rougher rides at high speeds as the boat is essentially

riding on a flat pad � much like a piece of plywood. Delta keels are susceptible to

porpoising. The delta keel also leads to �active steering� as flat surface provides little

directional stability. Therefore the driver must continually adjust the boats direction.

A full delta keel that comes to the bow will also slap the waves as this portion of the boat

repeatedly hit the flat portion of the keel.

To fairly compare dead rise between a true vee boat and a delta keel, the delta keel dead

rise should be cut at least in half or less to compare ride through the water. Thus a 16o

delta keel will ride closer to an 8o or less true vee. This is particularly true at higher

speeds where the ride will be roughest as only the stern portion (and flattest portion of the

delta keel) is in the water when underway.

Deadrise (Vee)

Wave cutting ability and ride is also influenced by the amount of Vee or deadrise of the

hull from front to back. Deadrise or Vee is usually measured in degrees.

Deadrise can be continuous or variable. The definition of deep vee generally ranges from

12o up to 25o. Once again another compromise arises. The deeper the Vee

� the more effectively the hull will cut through waves,

� the more (lateral) stability is decreased and

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� the power that is required to plane the hull is increased.

� the faster the planing or �stalling� speed is.

� the less load carrying capacity exists

There is no consistent method of measuring deadrise across the industry. The deadrise

measure that really counts is the deadrise at the back of the boat as this is where the boat

generally rides once it planes. Many manufacturers build a variable vee hull that tapers to

a small vee at the back. Although it may be advertised as a deep vee hull, this

measurement may be taken near the front. For example, a deep vee series may have a 350

bow entry which transitions to an 18o vee at the stern.

So how do you decide what degree of hull to get? As in most things in boating, it depends

upon what you will be doing. If you are always in big water then go 18o. If you are going

to do mostly river and some lake, then Marathon has found 12o to be a good compromise.

If you are doing mostly lake and some river you still may want to go 12o if you only go

on the lake in good weather. The reality for most boaters, despite all the hype about deep

vee boats, is that most boaters rarely see anything over 1-3� chop. These conditions

usually exist when it is blowing 15-20 mph. Most boaters do not go out in these

conditions, as it is too rough to wake board and water ski. Therefore why go deeper vee

and pay the power, speed and load carrying penalty? As well, if you and your family

wakeboard, air chair, or you have young kids learning to ski, you will find the planing

speed of a deep vee boat is too fast.

Most deep vees need 20-22 mph to plane, which is too fast for novice or intermediate

wakeboarders and skiers. A practical example might help. Does this dynamic sound

familiar?

� Your (spouse) is water skiing or wakeboarding

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� the boat is struggling to stay on plane at 20 mph

� your spouse signals to go faster

� the boat accelerates, comes up on solid plane and then rockets ahead (because

drag is now reduced)

� your spouse frantically signals to slow done

� you pull back on throttle and fall back to your struggling 20 mph semi plane

� the cycle continues and no one has a good time.

This is one of the reasons tournament ski boats are very flat. If you want to go 16.5 mph

you can go 16.5. Need another 1 mph? Nudge the throttle ahead. There is no transitioning

from slogging along to planing. You get the same thing operating a 120 boat. The boat is

easy to drive, you are not fighting to stay on plane and the 12o is more than adequate for a

smooth ride in flat and small chop water conditions.

When you do need to run across the lake in 3-5� waves you will find that angling into the

waves allows you to make at least as good progress as many supposedly deep vee hulls.

A lot of the time, that is because many supposedly deep vees have pretty flat vees/delta

keels at the transom for reasons discussed earlier. Therefore, when planning the business

end of the bottom is not running on a deep vee but rather on a flat pad. Most customers

who run in lakes tell us that the ride on the 12o hulls is much better than they had

expected.

Boats: What to Look For. �� .

Introduction.

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Welded Boats in General:

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Our Bias

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Shop for a Manufacturer First

Look in the used boat ads

Judging the Production Staff

Manufacturer Reputation

Buying a Welded Boat for the First Time

Know the Boat you want

There is no Perfect Boat

Take a test ride.

Acceleration out of the hole

Bottom Width and Engine Size and Type.

Turning ability.

Stability at Speed: Porpoising versus bow down ��.

The Hook.

Spray Line?.

Boat Materials: Aluminum versus Fiberglass .

Cost differences between glass and aluminum . �� .

Types of Aluminum. �� .

Material Thickness

A Word about Welding

Suck marks, caterpillar tracks

Folded, �Origami� Construction

Hull Design

Delta Keel.

Deadrise (Vee)

Planing Hull Bottom Types ��

Constant deadrise V (including modified V hulls)

Flat bottom hulls (8 degrees deadrise or less)

Bow Entry Types. ��

Shallow Entry

Sharp Entry

Radius Bottom �� .

Lift Forces Explained.

Lateral Stability During Planing Turns

Bottom Shape and Jet Intake

Planing Ability

Reverse Chines

Bottom Width: Flair is not everything

No Trim.

Hull Construction Standards

Shallow Water Operation

No Exposed Drive

Safety

Thrust line and Center of Gravity In line

Propeller Torque is Different than Impeller Torque

Propulsion Resistance.

Power Plants

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Derived from Impeller Curves.

RPM Curve Derivation

Conclusion

Hull Resistance.

Boat attitude..

Jet Drives

How jets work .

Impeller

Cut.

Cut, Speed and Load

Wear ring.

Nozzle

Stator.

Hard Anodized.

Cavitation .

Intake

Seals

Mixed flow

Axial flow.

Outboard jet:

Inboard jets:

So, what is the best jet?

Corrosion.....

UHMW...

Jet Diagnosis...

Rattle at Idle (V8 boats)

Speed too low for a Given RPM.

Poor acceleration or �holeshot�, combined with high top end

Definitions.

Porpoising .

Chine walk: 9

Ram effect .

Static thrust:

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Spiridonov S. E.

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JETCRAFT. ��. �� . ��. ��. �� . �� . (Boats: What to Look For . Buyer guide)